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A selection of interesting papers that were published in thetwo months before our press date in major journals mostlikely to report significant results in immunology.

Current Opinion in Immunology 2000, 12:1–7

Contents (chosen by)

1 Antigen recognition (Elliott)2 Innate immunity (Bonneville)3 Lymphocyte development (Kruisbeek)3 Immunological techniques (Liu)4 Immunity to infection (Bonneville)4 HIV (Rowland-Jones)5 Genetic effects on immunity (Casanova)5 Cancer (Walker)6 Transplantation (Wood and Bushell)7 Autoimmunity (Bonneville)

• of special interest•• of outstanding interest

Antigen recognitionSelected by Tim ElliottJohn Radcliffe Hospital, Oxford, UK

TCR-mediated internalisation of peptide-MHC complexesacquired by T cells. Huang JF, Yang Y, Sepulveda H, Shi W,Hwang I, Peterson PA, Jackson MR, Sprent J, Cai Z: Science1999, 286:952-954.

ANDClass II MHC/peptide complexes are released from APCand are acquired by T cell responders during specific anti-gen recognition. Patel DM, Arnold PY, White GA, Nardella JP,Mannie MD: J Immunol 1999, 163:5201-5210. •••• Significance: These papers describe how antigen specificCD8+ cytotoxic T lymphocytes (CTLs) (Huang et al.) and CD4+

T cells (Patel et al.) can acquire specific MHC–peptide com-plexes from their target cells. Huang et al. show that the pas-sively acquired target structures can then be recognised byneighbouring CTLs with the same specificity. Since both CD4+

and CD8+ T cells die when they are recognised by their fratri-cidal neighbours, these findings suggest a mechanism for theregulation of vigorous T cell responses to antigen and couldunderpin the phenomenon of T cell exhaustion. Findings: Passive acquisition of complexes of MHC class IIwith peptide by CD4+ T cells has been described previouslybut Huang et al. describe the same phenomenon for CD8+

CTLs for the first time. Both studies show that the transfer ofcomplexes is rapid (within 30 minutes of co-culturing targetswith responders) and dependent on the specificity of the TCRfor the complex. However, Patel et al. also show that recogni-tion of the complex by the TCR is important only to activate therecipient T cell and that uptake of MHC–peptide complexes isthen mediated by other factors — concanavalin mediated acti-vation of CD4+ T cells can bypass the requirement for specificrecognition in this phenomenon. This group also showed that

lipids and other transmembrane proteins (CD4 and the IL-2receptor) are transferred along with the MHC, suggesting thatthe mechanism of transfer is via direct fusion of ‘right-side-out’vesicles from donor to recipient cell. These issues were notaddressed in the study by Huang et al., although this groupshow the co-internalisation of the TCR and MHC into acidicvesicles of the recipient CTLs, which would be more consistentwith the endocytosis of ‘inside-out’ vesicles and their subse-quent fusion with the plasmalemma. The study by Huang et al.also shows clearly that, upon MHC–peptide transfer, the recip-ient T cells can be victims of fratricide and that, since the donorcells need to be cultured in high concentrations of peptide anti-gen (around 100 nM) before the recipient cells are sensitisedfor fratricide, passive transfer may be a way of switching off avigorous CTL response to a virus where the antigen load is high.

Human transporters associated with antigen processing(TAP) select epitope precursor peptides for processing in theendoplasmic reticulum and presentation to T cells. Lauvau G,Kakimi K, Niedermann G, Ostankovitch M, Yotnda P, Firat H,Chisari FV, van Endert PM: J Exp Med 1999, 190:1227-1239. •• Significance: Trimming of antigenic peptides in the endo-plasmic reticulum (ER) has been described before but itsimmunological relevance has not yet been established. Thispaper shows that, for two naturally occuring immunodominantviral epitopes, amino-terminal trimming in the ER is likely to be aprerequisite for their effective presentation to cytotoxic T lym-phocytes (CTLs).Findings: The immunodominant, naturally presented CTL epi-tope that is derived from the hepatitis B virus (HBV) core (cor-responding to residues18–27) has a high affinity for HLA-A2(0.5 nM) but binds to TAP weakly (1–10,000 nM) and is trans-ported correspondingly poorly. An amino-terminally extendedpeptide (with a serine added at the amino terminus), however,bound well to TAP (300 nM) and was efficiently transported butbound to HLA-A2 poorly (100 nM). When the long or the shortpeptides were expressed in cells as minigenes using recombi-nant vaccinia, infection of targets with the extended epitope ledto recognition by CTLs but infection with the naturally occurringepitope did not. Immunopurification of HLA-A2 from targetsinfected with the extended epitope confirmed that the aminoterminal amino acid had been removed and that the optimal18–27 peptide was the only one presented to CTLs in thesecells. Taken together these data indicate that efficiency of pre-sentation of this peptide is determined by the efficiency oftranslocation into the ER of an amino-terminally extended pre-cursor which is then further processed in the ER before it is pre-sented at the cell surface. A similar phenomenon was seen fora second HBV derived epitope (pol575–583) that was extend-ed by two amino acids at its amino terminus.

Complete sequence and gene map of the human major his-tocompatibility complex. MHC Sequencing Consortium:Nature 1999, 40:921-923.

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ANDMolecular dynamics of MHC genesis unraveled bysequence analysis of the 1,796,398-bp HLA class I region.Shiina T, Tamiya G, Oka A, Takishima N, Kikkawa E, Iwata K,Tomizawa M, Okuaki N, Kuwano Y, Watanabe K et al.: Proc NatlAcad Sci USA 1999, 96:13282-13287.

ANDThe chicken B locus is a minimal essential major histocom-patibility complex. Kaufman J, Milne S, Gobel TF, Walker BA,Jacob JP, Auffray C, Zoorob R, Beck S: Nature 1999,401:923-925. •••• Significance: Two Nature papers (which are accompaniedby an informative and amusing ‘News and Views’ article fromPeter Parham) describe the entire sequence of the human (3.6Mb) and chicken (92 kb) MHCs. Not only this a significantachievement in its own right, it also provides insight into two dif-ferent evolutionary strategies for developing an effectiveimmune system. Shiina et al. add some important observationson the evolution of the MHC class I region.Findings: The human and fowl MHCs diverged about 300 mil-lion years ago; during this time the amount of DNA in the formerhas increased and given rise to a region in which genes withknown immunological function, though densely packed com-pared to the rest of the genome, are interspersed with nonim-munological genes and pseudogenes. This gives rise to an MHCwithin which recombination can occur at a high rate (around 2%)and can give rise to new haplotypes that may confer a selectiveadvantage to its evolving host species in a particular pathogen-ic milieu. Shiina et al. shed some light on the possible mecha-nism by which the MHC class I region has expanded – via theduplication of segments of around 20 kb containing an MHCclass I gene and a class-I-related MIC gene. They postulate theancestral unit to be HLA-F and MIC-E. The paper by Kaufman etal. reveals the chicken MHC to be extremely compact, with littleroom for recombination as a way of diversification; this suggeststhat genes that work together for the purpose of antigen pre-sentation (for example MHC class II and DM, or MHC class Iand TAP) may have co-evolved. In humans, MHC class I genesand the TAP genes have become separated by over 1 Mb.Notably, the chicken MHC contains two genes likely to code fornatural-killer cell receptors that are encoded on an entirely dif-ferent chromosome (12) in the human.

Natural adjuvants: endogenous activators of dendritic cells.Gallucci S, Lolkema M, Matzinger P: Nat Med 1999,5:1249-1255. •••• Significance: Activated dendritic cells (DCs) are the onlyantigen-presenting cells capable of initiating a new T cellresponse because they deliver costimulatory signals in additionto antigen-specific information. Costimulatory molecules mustbe induced in DCs before they can activate naive T cells. Thispaper suggests that factors capable of activating DCs includemechanical manipulation. It shows that apoptotic cells are notimmunostimulatory, which would help to explain how the bodycould withstand large amounts of developmentally associatedapoptosis without provoking autoimmune responses.Furthermore it shows that necrotic cells are immunostimulatoryand that DCs would therefore respond to this form of cell death

that is indicative of danger, for example as a result of virus-induced lysis.Findings: Bone-marrow cells cultured in low concentrations ofGM-CSF and IL-4 contain large numbers of resting myeloidDCs expressing low levels of costimulatory molecules (CD90,CD95 and CD40). This study shows that 10 ng/ml bacterialLPS, 10 ng/ml interferon-γ or even aspiration with a pipette caninduce high expression of these molecules, thus increasing theimmunopotency of the DCs considerably. Different treatmentsinduced the different costimulatory molecules to varyingextents, which may turn out to have functional ramifications.Activated DCs presented antigens via a direct route (process-ing of endogenous antigens) and an indirect route (processingof internalised antigens). As a source of internalised antigen,debris from necrotic but not apoptotic cells was effective.

Innate immunitySelected by Marc BonnevilleInstitut de Biologie, Nantes, France

ββ-defensins: linking innate and adaptive immunity throughdendritic and T cell CCR6. Yang D, Chertov O, Bykovskaia SN,Chen Q, Buffo MJ, Shogan J, Anderson M, Schröder JM, WangJM, Howard OMZ, Oppenheim JJ: Science 1999,286:525-528.•• Significance: Defensins are antimicrobial peptides that con-tribute to host defense by disrupting bacterial cell membranes.This study shows that some defensins also promote recruitmentof dendritic and T cells to the site of infection, thus providing alink between innate and adaptive immune responses.Findings: β-defensins are a subset of microbicidal peptidesproduced by epithelial cells of the skin, kidney and bronchialmucosa. It is shown here that human β-defensin 2(HBD2) ischemotactic for cells expressing CCR6, a chemokine receptorpreferentially found on immature dendritic cells and memoryT cells. Accordingly chemotactic activity of HBD2 was blockedby anti-CCR6 antibodies and by inhibitors of chemokine recep-tor signalling such as pertussis toxin. Besides, binding of LARC(the chemokine ligand for CCR6) to its receptor was competedout by HBD2.

The Toll-like receptor 2 is recruited to macrophage phago-somes and discriminates between pathogens. Underhill DM,Ozinsky A, Hajjar AM, Stevens A, Wilson CB, Bassetti M,Aderem A: Nature 1999, 401:811-815.

ANDDifferential roles of TLR2 and TLR4 in recognition of Gram-negative and Gram-positive bacterial cell wall components.Takeuchi O, Hoshino K, Kawai T, Sanjo H, Takada H, Ogawa T,Takeda K, Akira S: Immunity 1999, 11:443-451.•• Significance: Receptors belonging to the Toll family partici-pate in Drosophila innate immunity against bacteria and fungi.Several mammalian homologues of Drosophila Toll, called Toll-like receptors (Tlr), have been described but their specificity formicrobial products has remained controversial. These two stud-ies show that human and mouse TLR2 and TLR4 recognize dis-tinct sets of microbial and fungal components and may allowtriggering of inflammatory responses appropriate for defenceagainst a specific pathogen.

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Findings: The study of Underhill et al. shows that expression ofa dominant negative form of human TLR2 in a macrophage cellline abrogates inflammatory response to yeast and Gram-posi-tive bacteria but not to Gram-negative bacteria or lipopolysac-charide (LPS). Conversely, inflammatory response toGram-negative bacteria or LPS but not the Gram-postive bac-terial components are prevented in macrophages expressing adominant negative form of Tlr4. Takeuchi et al. have studiedresponses of macrophages derived from Tlr2- and Tlr4-deficientmice to LPS or to the Gram-positive components peptidoglycan(PG) and lipoteichoic acid (LTA). Tlr2-deficient macrophagesresponded normally to LPS but not to Gram-positive bacterialcell walls and PG whereas Tlr4-deficient macrophages werehyporesponsive to LPS and LTA but showed almost normalresponse to Gram-positive bacterial cell walls.

Lymphocyte developmentSelected by Ada KruisbeekThe Netherlands Cancer Institute, Amsterdam, The Netherlands

Ig-αα cytoplasmic truncation renders immature B cells moresensitive to antigen contact. Kraus M, Saijo K, Torres RM,Rajewsky K: Immunity 1999,11:537-545.• Significance: The antigen receptor on B cells (BCR) triggerssurvival, differentiation and expansion signals. It does sothrough the BCR-associated Ig-α and Ig-β invariant chains,which each contain a single immunoreceptor tyrosine activationmotif (ITAM) in their cytoplasmic tail that can couple — whenphosphorylated — to SH2-domain-containing proteins. Thisevent then initiates further downstream signaling cascades.Earlier studies established a crucial positive regulatory role forthe ITAM in the Ig-α chain, in that mice lacking the cytoplasmictail of Ig-α have a severe block in early B cell development. Thepresent study provides evidence that Ig-α can also have a neg-ative regulatory role during B cell development.Findings: Mutant mice lacking the cytoplasmic tail of Ig-α (mb-1∆c/∆c mice) were crossed to mice expressing a transgenic BCRspecific for hen egg lysozyme (HEL) and co-expressing eithersoluble or membrane bound HEL (sHEL or mHEL). In themHEL-expressing mice, the outcome was comparable for wild-type and mutant Ig-α mice: in the bone marrow, clonal deletionof B cells expressing transgenic anti-HEL BCRs occurred. Thisdocuments that the Ig-α tail is not required for negative selec-tion. Quite unexpectedly, immature B cells in mice expressingsHEL and HEL-specific BCRs undergo rearrangements ofendogenous κ light chain genes in the absence of Ig-α; HEL-specificity is lost as a consequence and, since absence of Ig-αis correlated with hyper-responsiveness upon BCR crosslinking,it is suggested that Ig-α has a negative regulatory role in imma-ture B cells, besides having an activating role in mature B cells.

αα4 integrins regulate the proliferation/differentiation bal-ance of multilineage hematopoietic progenitors in vivo.Arroyo AG, Yang JT, Rayburn H, Hynes RO: Immunity 1999,11:555-566. • Significance: As hematopoiesis requires migration of pluripo-tent stem cells, interactions of these progenitors with themicroenvironment play a crucial role. The α4 integrins havebeen implicated in such interactions and a role for α4 integrins

in erythropoiesis has been suggested in a number of studies.This report addresses whether α4 integrins also have a role inmyeloid and lymphoid cell development.Findings: Analysis of α4-null mutant and chimeric mice duringembryonic development indicate that yolk-sac erythropoiesisand migration of hematopoietic progenitors to fetal liver, spleenand bone marrow are not dependent on α4 integrins: at allembryonic stages analyzed, the thymus of chimeric embryoscontained comparable numbers of α4-deficient and wild-typethymocytes, and these develop normally. In sharp contrast,B lymphoid and myeloid differentiation from α4-deficient prog-enitors in α4-deficient chimeric embryos is significantlyreduced. These defects are extrapolated postnatally: in theabsence of α4 integrin, myeloid and B lymphoid developmentare severely compromized and erythropoiesis is also affected atthis stage. It appears that the progenitors of these lineageshave defects in transmigration through stroma, but it is interest-ing that this only affects their ability to proliferate and differenti-ate in the fetal liver, bone marrow and spleen but it does notaffect their ability to develop in the thymus microenvironment.

Defective thymocyte maturation in p44 MAP kinase (Erk 1)knockout mice. Pagès G, Guérin S, Grall D, Bonino F, SmithA, Anjuere F, Auberger P, Pouysségur J: Science 1999,286:1374-1377.• Significance: Erk1 (a mitogen-activated protein kinase[MAPK]) and its isoform Erk2 have been implicated in the sig-naling cascades induced by a broad range of receptors, in vir-tually all cell types. They are activated through the Ras–Raf–MEK cascade, but it remains to be determined whether theyhave specific roles. As earlier studies suggested that the MAPKcascade may contribute to thymocyte differentiation, this studyexamines, for the first time, whether deletion of Erk1 has impacton T cell development.Findings: Erk1-knockout mice were generated and found to beviable, fertile and of normal size. In the thymus of these mice,there is a clear reduction of single-positive (SP) mature CD4+

CD8– and CD8+ CD4– thymocytes, and a concomitantincrease in immature double-positive (DP) thymocytes. Erk1-deficient thymocytes also have a significant defect in TCR-dri-ven thymocyte expansion, while the apoptotic response ofthymocytes is said not to be affected. While further relevantinformation is lacking at the present time (it would have beenquite relevant to know at which point in the multistep programof positive selection the block appears; also information on pre-T-cell development would be relevant), this phenotype doessupport the notion of a special role for Erk1 that cannot be com-pensated by Erk2.

Immunological techniquesSelected by Yang LiuOhio State University, Columbus, Ohio, USA

Activation changes the spectrum but not the diversity ofgenes expressed by T cells. Teague TK, Hildeman D, KedlRM, Mitchell T, Rees W, Schaefer BC, Bender J, Kappler J,Marrack P: Proc Natl Acad Sci USA 1999, 96:12691-12696.• Significance: This is the first systematic comparison betweenthe spectrum of genes expressed in resting T cells and that of

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genes expressed in T cells at either 8 or 48 hours after engage-ment of the TCR. The observation that a surprisingly large num-ber of genes are expressed in resting T cells supports thenotion that the state of resting T cells is not a passive failure torespond to extant external stimuli.Findings: Using the Affirmatrix gene-array chip system, theinvestigators analyzed expression of approximately 6300 genesin resting and activated T cells. They found that diversities ofgene expression are similar among resting and activated T cells.

Persistence of memory CD8 T cells in MHC class I-deficientmice. Murali-Krishna K, Lau LL, Sambhara S, Lemonnier F,Altman J, Ahmed R: Science 1999, 286:1377-1381.

ANDClass II-independent generation of CD4 memory T cellsfrom effectors. Swain SL, Hu H, Huston G: Science 1999,286:1381-1383. •• Significance: These two groups used MHC-deficient hostsas recipients for adoptive transfer studies. The results challengethe current dogma that the MHC is required for survival of allT cells and suggest that memory cells and naive T cells havedistinct requirements for their survival and expansion.Findings: The authors demonstrated that survival and expan-sion of memory T cells are independent of the restricting MHCclasses and loci. The authors of the second study also sug-gested that generation of CD4+ memory cells from effectorT cells is also independent of MHC class II.

Live cell fluorescence imaging of T cell MEKK2: redistributionand activation in response to antigen stimulation of the T cellreceptor. Schaefer BC, Ware MF, Marrack P, Fanger GR, KapplerJW, Johnson GL, Monks CRF: Immunity 1999, 11:411-421. •••• Significance: Using live T cells that were transduced withgreen-fluorescence protein (GFP)-tagged MEKK2, the dynam-ics of the translocation of this critical kinase were visualizedusing fluorescence microscopy. This novel approach shouldfind wide-spread applications in studying the dynamic assemblyof the signaling machines within the cells.Findings: Engagement of the TCR by its specific antigenic epi-tope on antigen-presenting cells leads to rapid translocation ofMEKK2 and formation of a MEKK2-containing signaling com-plex. Disruption of this process inhibits T cell activation.

Immunity to infectionSelected by Marc BonnevilleInstitut de Biologie, Nantes, France

Extraintestinal dissemination of Salmonella by CD18-expressing phagocytes. Vazquez-Torres A, Jones-Carson J,Baumler AJ, Falkow S, Valdivia R, Brown W, Le M, Berggren R,Parks WT, Fang FC: Nature 1999, 401:804-808. •• Significance: This study provides the first strong evidencethat induction of systemic or mucosal immunity against gastrointestinal pathogens depends on the route of bacterialuptake. This observation should have important implicationsfor the design of oral vaccines aiming at inducing systemicimmune responses. Findings: M cells are thought to play a key role in the inductionof mucosal immune responses against gastrointestinal

pathogens, by permitting transepithelial transport of luminalantigens that are then exposed to subjacent follicular leuko-cytes. This study shows that Salmonella strain mutants that areno longer able to invade M cells can disseminate to the liver orthe spleen and trigger IgG1 (systemic) but not IgA (mucosal)responses. Dissemination of Salmonella to the bloodstream ismediated by phagocytes expressing CD18, a β2-integrininvolved in leukocyte transmigration, and does not occur inCD18-deficient mice.

The immunoevasive function encoded by the mousecytomegalovirus gene m152 protects the virus againstT cell control in vivo. Krmpotic A, Messerle M, Crnkovic-Mertens I, Polic B, Jonjic S, Koszinowki UH: J Exp Med 1999,190:1285-1295. • Significance: This report provides the first formal proof thatcytomegalovirus (CMV)-derived proteins downmodulatingMHC molecule expression protect the virus against CD8+ T cellimmunity in vivo. Findings: The mouse CMV protein gp40, encoded by them152 gene, inhibits antigen presentation by the MHC class Ipathway in vitro by blocking transport of MHC class I complex-es to the cell surface. It is shown that whereas deletion of them152 gene in CMV recombinants has no effect on viral repli-cation in vitro, m152-deficient virus replicates to lower titersafter infection of mice (compared to wild-type virus or m152gene revertants). By contrast, deficient mutants and revertantsgrow to the same titers in animals deficient for β2-microglobu-lin or CD8 molecules. These results demonstrate that deletionof the m152 gene results in a higher susceptibility of the virusto CD8+ T cell control.

A universal influenza A vaccine based on the extracellulardomain of the M2 protein. Neirynck S, Deroo T, Saelens X,Vanlandschoot P, Jou WM, Fiers W: Nat Med 1999,10:1157-1163.• Significance: The antigenic variation of immunogenic proteinsfrom influenza virus has hampered generation of a broad-spec-trum vaccine against this virus. This study describes for the firsttime a vaccine that provides long-lasting protective immunity inmice against diverse influenza A (infA) virus strains.Findings: Unlike the highly immunogenic hemagglutinin andneuraminidase proteins of the infA virus, the extracellulardomain of the M2 protein is nearly invariant in all infA strains. Inorder to enhance its immunogenicity, this M2 domain was fusedto the hepatis B virus core (HBc) protein, which has proven itsefficacy in eliciting high-titer antibody responses to heterolo-gous epitopes. Intraperitoneal or intranasal administration ofrecombinant M2–HBc particles produced in Escherichia coliprovided an almost complete protection against a lethal chal-lenge with various infA strains; protection lasted at least half ayear after immunization.

HIVSelected by Sarah Rowland-JonesJohn Radcliffe Hospital, Oxford, UK

Virus-specific cytotoxic T-lymphocyte responses select foramino-acidvariation in simian immunodeficiency virus env

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and Nef. Evans DT, O’Connor DH, Jing P, Dzuris JL, Sidney J,da Silva J, Allen TM, Horton H, Venham JE, Rudersdorf RA et al.:Nat Med 1999, 5:1270-1276.• Significance: This report provides clear evidence for selec-tion of variant viruses that can escape cytotoxic T cell recogni-tion following infection with SIV in a family of rhesus macaquesof defined MHC haplotype.Findings: Although strong circumstantial evidence has sug-gested in the past that selection pressure from CTLs leads tothe emergence of escape variants of both SIV and HIV, formalproof has been harder to obtain. In these studies, five relatedmacaques — who shared a common class II MHC haplotype —were infected with the same stocks of SIV. Two of the animalsprogressed to disease without mounting an effective CTLresponse; the epitopes recognised by CTLs from the other ani-mals were mapped in detail. Over time, there was an accumu-lation of mutations in the viral sequences in each animal at sitesof CTL recognition: evidence of selection was provided by anincreased ratio of non-synonymous to synonymous mutations atthese sites. The epitope variants that resulted generally led toloss of CTL recognition.

Sexual transmission and propagation of SIV and HIV inresting and activated CD4(+) T cells. Zhang Z, Schuler T,Zupancic M, Wietgrefe S, Staskus KA, Reimann KA, ReinhartTA, Rogan M, Cavert W, Miller CJ et al.: Science 1999,286:1353-1357. •• Significance: This paper shows that the predominant site ofSIV and HIV replication, both at the portal of entry and in lym-phoid tissue, is in CD4+ T cells. Moreover a significant reservoirof virus is found in resting CD4+ T cells, which persists afterantiretroviral therapy.Findings: This paper challenges the previously accepted viewthat the prime site of early HIV replication is in mucosal den-dritic cells and macrophages. Tissues taken from experimental-ly infected macaques shortly after infection were examined by insitu hybridisation for cells with detectable SIV RNA. The vastmajority of infected cells were CD4+ T cells and a surprisingfinding was that a small population of these T cells lackedexpression of Ki67 and HLA-DR, consistent with their beingresting T cells. This population persists after antiretroviral thera-py, suggesting that it constitutes an important reservoir of HIVin the treated patient.

Weak anti-HIV CD8(+) T-cell effector activity in HIV primaryInfection. Dalod M, Dupuis M, Deschemin JC, Goujard C,Deveau C, Meyer L, Ngo N, Rouzioux C, Guillet JG, DelfraissyJF et al.: J Clin Invest 1999, 104:1431-1439. • Significance: This report suggests that the range and magni-tude of the HIV-specific CD8+ T cell response are much weak-er in primary infection than expected on the basis of theresponse seen in established chronic infection.Findings: The authors used IFN-γ ELISpot assays to look at theCD8+ T cell responses to a variety of known CTL epitope pep-tides in 24 patients with acute HIV infection in comparison with30 HIV-infected subjects at later stages of disease. In generala much smaller range of peptide-specific responses as detect-ed in primary infection (range 0–6, median 2) than in chronical-

ly infected patients (range 1–13, median 5). The magnitude ofthe response was also lower in primary infection than has beenobserved in acute EBV (Epstein–Barr virus) infection in humansand LCMV (lymphocytic choriomeningitis virus) infection in themouse. This suggests that the CTL response to acute HIVinfection, which has been thought to play an important role inthe initial control of viral replication, nevertheless appears to berelatively blunted.

Genetic effects on immunitySelected by Jean-Laurent CasanovaInstitut National de la Sante et de la Recherche Medicale, HopitalNecker-Enfants Malades, Paris, France

Mutations in Igαα (CD79a) result in a complete block in B-cell development. Minegishi Y, Coustan-Smith E, Rapalus L,Ersoy F, Campana D, Conley ME: J Clin Invest 1999,104:1115-1121. •• Significance: This is the first identification of a human inherit-ed defect in a component of the signal-transduction module ofthe pre-B-cell-receptor complex.Findings: A child with unexplained agammaglobulinemia andabsent B cells was investigated. A homozygous splice mutationwas identified in the Igα gene that precluded expression of awild-type transcript in the bone marrow. By studying cell surfaceand intracellular markers of differentiation, as well as the diver-sity of the immunoglobulin µ-heavy-chain variable-region VDJrepertoire, the block in B cell differentiation in the patient wasshown to be identical to that seen in other patients with defectsin the µ-heavy-chain constant region. This remarkable study notonly describes a novel human inheritable immune disorder butalso indicates that Igα does not play a role in human B celldevelopment until it is expressed along with µ heavy chain aspart of the pre-B-cell-receptor.

CancerSelected by Paul R WalkerUniversity Hospital Geneva, Geneva, Switzerland

Chronic brain inflammation and persistent herpes simplexvirus 1 thymidine kinase expression in survivors of syn-geneic glioma treated by adenovirus-mediated gene thera-py: implications for clinical trials. Dewey RA, Morrissey G,Cowsill CM, Stone D, Bolognani F, Dodd NJ, Southgate TD,Klatzmann D, Lassmann H, Castro MG, Lowenstein PR: NatMed 1999, 5:1256-1263.•• Significance: Gene therapy for glioma using herpes-simplex-virus-1 thymidine kinase (HSV-1-TK) delivered by adenoviral vec-tors is currently being actively pursued in clinical trials. Bothtransduced and neighbouring, nontransduced cells can be killedby the direct effects of ganciclovir administration or by nonspe-cific ‘bystander effects’. However, this paper describes possiblelong-term consequences that need to be considered — long-last-ing gene expression in wide areas of the brain and chronic wide-spread inflammation. These factors need thorough assessmentfor optimising treatment strategies whilst minimising inflamma-tion that could eventually compromise brain function. Findings: Implantation of CNS-1 glioma cell line into the brainof syngeneic rats results in progressive tumour growth anddeath of all animals within 30 days. If rats are treated with HSV-

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1-TK followed by ganciclovir, 80–100% survive for at least 3months and generally show no detectable tumour at the time ofsacrifice. Histopathological analysis showed widespread reac-tive gliosis and leukocyte infiltration, with localised demyelina-tion. Furthermore, contrasting with many previous studies, therewas widespread, long-term persistence of vector genome andimmunoreactive HSV-1-TK. The authors suggest that the per-sistent inflammation is a consequence of HSV-1-TK and ganci-clovir although it is clear that during the treatment manyundefined immune responses are occurring that may contributeto the observed pathology.

Effect of costimulation and the microenvironment on anti-gen presentation by leukemic cells. Buggins AG, Lea N,Gaken J, Darling D, Farzaneh F, Mufti GJ, Hirst WJ: Blood 1999,94:3479-3490.•• Significance: Modifying tumour cells by expression of co-stimulatory molecules such as CD80 (B7.1) is an attractiveapproach to potentially improve T cell responses to tumour anti-gens. However the consequences for the overall antitumourresponse have been variable, with insufficient understanding ofthe direct effects on T cells receiving costimulation by thetumour. This paper makes a commendable effort to addressthese issues and concludes that, in the model used, T cells areefficiently stimulated but do not express the optimal cytokinesfor an efficacious antitumour response.Findings: The human monocytic leukaemia cell line U937 wastransfected with CD80 (U937-CD80) and used to stimulateallogeneic T cells. The threshold of TCR stimulation required foractivation was reduced by the presence of CD80 on U937, afeature potentially useful to elicit responses to tumour antigenswith a low expression level. However, U937-CD80 cells stimu-lated little or no Th1 cytokine release (interferon-γ or IL-2) byallogeneic T cells. Inhibition of Th1 cytokine release was medi-ated by U937-derived soluble factors that were not identifiedbut did not correspond to TGF-β, IL-10 or prostaglandin E2.

A proinflammatory cytokine inhibits p53 tumor suppressoractivity. Hudson JD, Shoaibi MA, Maestro R, Carnero A,Hannon GJ, Beach DH: J Exp Med 1999, 190:1375-1382. •• Significance: Mutation of the p53 tumour suppressor gene isthe most frequent genetic alteration in human tumours and maybe central to tumorigenesis. Regulation of p53 is thus a criticalparameter to understand. This paper by Hudson et al. now pro-vides mechanistic details of how p53 function may be altered bymacrophage migration inhibitory factor (MIF). The findings mayexplain the previously observed associations between chronicinflammatory states and the appearance of certain tumours. Findings: Using different functional tests, MIF was identified as agene product that was capable of over-riding p53-mediatedgrowth arrest by suppressing its transcriptional activation func-tion. Furthermore MIF was shown to suppress p53-dependentapoptosis, possibly reflecting its normal physiological role in pro-tecting macrophages from the proapoptotic effects of nitric oxidereleased in inflammatory sites. However these effects, togetherwith other findings such as the prolongation of the lifespan of pri-mary cells in culture — may allow cells to proliferate in a microen-vironment in which DNA damage is induced, permitting the

eventual accumulation of oncogenic mutations. Without a reduc-tion in inflammation and MIF levels, cellular transformation andtumour development may be the consequence.

Increased vaccine-specific T cell frequency after peptide-based vaccination correlates with increased susceptibilityto in vitro stimulation but does not lead to tumor regression.Lee K-H, Wang E, Nielsen M-B, Wunderlich J, Migueles S,Connors M, Steinberg SM, Rosenberg SA, Marincola FM:J Immunol 1999, 163:6292-6300.•• Significance: Quantitative assessment of the immunologicaloutcome of tumour vaccination protocols is essential to attemptto try and understand any correlation with clinical outcome. Thisis the first study in which the specific immune response elicitedby epitope-specific tumour vaccination has been directly moni-tored by MHC–peptide tetramers. Findings: Melanoma patients were immunised with natural ormodified gp100 peptides, with either adjuvant, IL-12 or IL-2.Melanoma-antigen-specific T cells were quantified withMHC–tetramers, with samples from peripheral blood and in afew cases from the tumour. Samples were analysed soon afterisolation and again after in vitro expansion. Although immunisa-tion generally increased specific precursor frequencies, thenumbers of specific cells were modest and — in the case ofpeptide and IL-2 administration — frequencies sometimesdecreased. However the specific cells were highly proliferativein vitro. Paradoxically, positive clinical responses were associat-ed with regimes in which cytotoxic T lymphocyte (CTL) fre-quency actually diminished. In certain cases this may be due tothe fact that CTLs in blood do not necessarily reflect those pre-sent in the tumour bed but, for the majority of cases, this find-ing remains an enigma. Tetramer technology is a powerful toolbut will not be the sole key to understanding the complexity ofantitumour immune responses.

TransplantationSelected by Kathryn Wood and Andrew BushellJohn Radcliffe Hospital, Oxford, UK

The functional relevance of passenger leukocytes andmicrochimerism for heart allograft acceptance in the rat.Ko S, Deiwick A, Jager MD, Dinkel A, Rhode F, Fischer R, TsuiT-Y, Rittman KL, Wonigeit K, Schlitt HJ: Nat Med 1999,5:1292-1297.•• Significance: In recent years the role of chimerism ormicrochimerism in determining graft outcome in both clinicaland experimental transplantation has been an area of activedebate. Supporters of the chimerism theory propose that thelong-term persistence of donor-derived cells (probablyhematopoietic cells carried over in the graft itself) contribute toa state of active donor-specific tolerance and thus to allograftsurvival. The alternative view is that these cells survive simplybecause of the immunosuppression given to the recipient andthat their presence is merely an epiphenomenon and has verylittle to do with graft outcome. The debate has continued forseveral years because it has been difficult to provide definitiveevidence either supporting or refuting the hypothesis. Findings: Ko et al. took advantage of the di-allelic polymor-phism of the rat leukocyte common molecule CD45 and of the

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availability of a monoclonal antibody specific for the RT7a allo-type. When lymph node cells from LEW. RT7a rats were inject-ed into congeneic LEW.7B (RT7b) rats, donor cells could bedetected in the peripheral blood for at least 28 days (about 5%of the total cells). When these reconstituted rats were given theanti-RT7a antibody this figure fell to <0.1%, demonstrating theability of the antibody to deplete donor cells selectively.LEW.7B (RT7b) rats were then transplanted with cardiac allo-grafts from fully MHC incompatible LEW.1W (RT7a) donorsunder the cover of a 14-day course of cyclosporine and weregiven the anti-RT7a antibody either at day 0 or at day 18 todeplete donor passenger leukocytes. Control animals receivedcyclosporine only. Graft outcome was similar in all treatmentgroups (median survival time >200 days) but a significant dif-ference was seen on histological examination of the grafts.Hearts from the cyclosporin-only and the cyclosporin plus day-18 antibody groups were practically identical in that they hadextremely good myocardial preservation and few signs of vas-culopathy. In clear contrast, hearts in the group depleted ofdonor passenger cells at the time of transplant had a significantT cell infiltration and severe vasculopathy. These observationsconfirm similar conclusions obtained previously in an alternativemodel of allograft tolerance (A Bushell et al., Transplantation1995, 59:1367-1371.) and support the view that although anti-gen-specific tolerance requires the presence of donor derivedcells, the requirement is only for short-term antigen persistencerather than for stable long-term microchimerism.

Blocking both signal 1 and signal 2 of T cell activation pre-vents apoptosis of alloreactive T cells and induction ofperipheral allograft tolerance. Li Y, Li XC, Zheng XX,Wells AD, Turka LA, Strom TB: Nat Med 1999 5:1298-1302.•• Significance: Blockade of the CD28–B7 and CD40–CD40Lcostimulation pathways, either alone or in combination, hasproved to be very effective in allowing long-term allograft sur-vival in a number of rodent models. Furthermore, when adminis-tered as a monotherapy, anti-CD40L (anti-CD154) antibodyhas given encouraging results in both kidney and islet trans-plantation a clinically relevant rhesus monkey model. Howeverseveral studies have indicated that – when combined withcyclosporin – the impressive effects of anti-CD40L antibody aresignificantly diminished, often leading to acute rejection. Thereis obviously a pressing need to understand the reasons for thissurprising phenomenon and to identify immunosuppressiveagents that augment rather than diminish the effectiveness ofcostimulation blockade.Findings: Mouse heart transplants were performed in theBALB/c (H-2d) to C3H (H-2k) strain combination. Recipientsgiven CTLA4-Ig plus anti-CD40L antibody accepted their graftsindefinitely but addition of a 14-day course of cyclosporin to thesame protocol led to acute graft rejection (median survival time30 days). In striking contrast, addition of rapamycin to theCTLA4-Ig plus anti-CD40L antibody protocol had no effect oncostimulation blockade — all animals accepted their grafts long-term. In order to identify any additive effect of costiumlationblockade and rapamycin, Li et al. transplanted full-thickness skingrafts in the same strain combination. Costimulation blockadewas ineffective in their hands but the addition of rapamycin led

to permanent skin graft survival. Adoptive transfer of CFSE-labelled syngeneic cells followed by antigen-challenge showedthat costimulation blockade resulted in reduced proliferationand an increase in the overall rate of antigen-induced apopto-sis. This was completely inhibited in the presence ofcyclosporin but substantially enhanced by rapamycin. The con-clusion was that although cyclosporin appears to inhibit theeffectiveness of costimulation blockade by preventing activationinduced cell death, rapamycin promotes the apoptosis ofdonor-reactive cells. Overall, these data provide more supportfor the notion that if costimulation blockade is to be introducedinto clinical practice in combination with conventional immunop-suppression, rapamycin and not cyclosporin may be the agentof choice.

AutoimmunitySelected by Marc BonnevilleInstitut de Biologie, Nantes, France

Autoreactive CD4+ T cells protect from autoimmune dia-betes via bystander suppression using the IL-4/Stat6 path-way. Homann D, Holz A, Bot A, Coon B, Wolfe T, Petersen J,Dyrberg TP, Grusby MJ, von Herrath MG: Immunity 1999,11:463-472.•• Significance: This study provides important insights into themechanism of ‘bystander suppression’ in an autoimmune type Idiabetes model. It also proposes a promising therapeutic strat-egy using autoreactive regulatory cells that does not requireprecise knowledge of the primary, eliciting self antigen. Findings: Regulatory cells targeted to affected organs can sup-press destructive autoimmune processes through release ofimmunosuppressive cytokines such as IL-4, IL-10 or TGF-β. Theprecise mechanism of this ‘bystander suppression’ remains elu-sive. This study shows that autoreactive regulatory T cells spe-cific for insulin B chain, that are induced by oral administration ofthe recombinant antigen, can efficiently suppress destructiveautoimmunity mediated by T cells directed against a different selfantigen. This effect is mediated by IL-4, which affects the abilityof antigen-presenting cells to activate autoaggressive T cells.

Activated T cells regulate bone loss and joint destruction inadjuvant arthritis through osteoprotegerin ligand. Kong YY,Feige U, Sarosi I, Bolon B, Tafuri A, Morony S, Capparelli C,Elliott R, McCabe S, Wong T et al. Nature 1999, 402:304-309. •• Significance: This study provides the first strong evidencefor an important role for T cells as regulators of bone home-ostasis through production of osteoprotegerin ligand (OPGL).It also provides a new therapeutic target for treatment of boneand cartilage destruction occurring during severe arthritis. Findings: This study shows that activated T cells can directlytrigger osteoclastogenesis through OPGL in vitro. Transfer ofbone marrow cells derived from ctla4-deficient mice into lym-phocyte-deficient hosts, which results in massive systemic pro-duction of spontaneously activated T cells, leads to anOPGL-mediated increase in osteoclastogenesis and bone loss.In a rat model of acute adjuvant-induced arthritis mediated byT cells, blocking of OPGL function through treatment with thedecoy receptor osteoprotegerin at the onset of the diseaseinhibits bone and cartilage erosion.

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