Immunology Today, voL 8, No. 1 I, 1987 , =1

@

The molecules controlling B lymphocytes

The need to activate B lymphocytes on antigen challenge is tempered by the requirement that the response is terminated when the challenge has been negotiated. Regulation is prob- ably mediated via the interaction of soluble factors with surface receptors. Here, John Gordon and Graeme Guy review recent work that has focused on the actions of the soluble regulatory mediators and on the surface molecules they may interact with

- emphasizing the growing importance of the CD23 antigen.

Perhaps the most impressive feature of the immune system is the potent homeostasis which ensures that a rapid and vigorous response to specific antigen challenge is followed by an equally efficient return to quiescence once that challenge has been met. Deciphering the various elements which determine this strict and neces- sary control continues to be a major quest for immuno- logists. Classically, attention has been focused on such components as the complement cascade and the idio- typic network, but now it is the cellular arms that are receiving particular scrutiny for their contribution to immune regulation. This review, concentrating on the human system, highlights those recent advances which have led to a more precise understanding of the proces- ses controlling the response of B lymphocytes to antigen. The emphasis will be on the soluble factors and surface molecules involved. It is worth remembering that what has become a frenetic (and, for many, bewildering) area of research started little more than a decade ago with the simple observation that the soluble products of T I v n n n h n , ~ t c = ¢ r r ~ , I N ¢ , , h e * i f , , * - , for ,k~ ,-~,11~ + i ~ , , ~ , , - ^ h , , . ~ - " ",I " " " r " " .V~ . l r b ~ , . , . v u | u . ~ u u . . i l . i s.u ¢'q,.,. ¢11~ %..'~llJ I.I I1~"11 I , ,~ I~IVC3 i n

certain T-cell-dependent B-cell responses ~. Only in the past year or so has there been any substantive advance on that original report.

The rapid progress towards unravelling the complex- ities of B-lymphocyte growth can be attributed to three key developments. Undoubtedly the most critical of these has been the availability of cytokines in recom- binant form. Also important has been the realization that many distinct activation states are available to the B lymphocyte, emphasizing the need for homogeneous, well-characterized target populations in functional assays. Finally, the International Workshops on Human Leucocyte Differentiation Antigens 2 must be acknow- ledged for their contribution in defining the monoclonal antibodies which have subsequently been used to probe the potential receptor function of B-cell surface struc- tures.

B-cell growth factors Some of the major properties of the human recom-

binant factors which have been claimed to act on B cells are summarized in Table 1. A single glance will make it apparent that B-cell growth factors are neither tissue specific in their target range nor stage specific for their a~ion on B lymphocytes. Furthermore, single events in

De,~rtment of Immunobgy, The Medical School, Vincent Drive, Birmingham B15 2TJ, UK.

1987, Elsevier Publications, Cambridge 0 ! 6 7 - 4 9 1 9 / 8 7 / $ 0 2 . 0 0

the growth and differentiation pathways appear to be guided by many factors. Thus, both the specificity and exclusivity which had previously been demanded of a B-cell growth factor are no longer in evidence. The control, required and observed, in the B-cell programme must therefore reside in levels other than those operat- ing through the direction of single factors. To consider these concepts in detail, it is necessary to examine closely the stages of B-cell activation and growth cycles, their associated phenotype and the ways they can be reached with defined stimuli.

Broadly speaking, the B-cell response to antigen has been considered in three major waves with the biological properties of the cytokines classified accordingly (Fig. 1). Thus, one class of factor has been described by its ability to synergize with a suboptimal dose of anti- immunoglobulin (anti-lg) in provoking the entry of rest- ing B cells into the S phase of cycle. Such activities have generally been termed B-cell growth factor I (BCGF-I). This nomenclature implies that S phase entry necessarily equals growth and that these factors have to act within the growth cycle. It has become clear, however, that some mediators are able to drive activated B cells through S and into the G2 phase of the cell cycle without the cells proceeding to mitosis, while others, which have potent anti-lg co-stimulator activity, perturb B cells be- fore they have entered the growth cycle proper. Candi- ,,o~'̂ ~: molecul~ for the =,u,m~, act~v~y (i.e. progression factors) include a 12kDa T-cell derived BCGF for human B-cell stimulations 3 (often referred to as low molecular weight BCGF), the C3d component of complement for cycling murine B cells 4 and interleukin 1 (IL-1), at least as a co-factor, in both species 5.6. By contrast, interleukin 4 (IL-4) represents a BCGF-I which can act, at least on murine B cells, before they enter the cell cycleT; hence its preferred synonym of B-cell stimulatory factor 1 (BSF-1).

Factors having a direct effect on growth (i.e. replica- tion) tend to be classified as 'BCGF-II' activities. The major assay for such cytokines has been their ability to support the growth of murine BCL1 tumour cells ex- planted to tissue culture s . For human factors, assays have been additionally based on the restimulation of cells previously stimulated for three days with Staphylococcus aureus Cowan Strain I (SAC) and on the continued synthesis of DNA in low buoyant density (considered pre-activated) tonsillar B cells 9. Human molecules de- scribed as replication factors include a high molecular weight (50-60 kDa) B-cell growth factor derived both from T cells and from the Namalwa B-cell line 1°, inter- leukin 2 (IL-2) 11, and gamma interferon (IFN-~/) 12. Whether high molecular weight BCGF is equivalent to interleukin 5 (IL-5)is unclear. While IL-5 has been claimed to act as a replication factor for murine B cells ~3, the human recombinant form, which has excellent eosino- phil differentiation activity, has so far failed to register positive in all conventional B-cell assays (C.J. Sanderson 339

Immunology Today, vol. 8, No. 1 I, 1987

TaMe 1, Recoml~nant human molecules w~th growth and differentiation activity for B lymphocytes

IL-la IL-11~ IL-2 IL-4 IL-5 BSF-2

BCGF IFN-a IF'N-'y

Lymphocyte activating factor (LAF) 17 (32)

T-cell growth factor (TCGF) 15 BS~I, BCGF-I, BCDFe, BCD~ 20 T-c~ll ~=placing factor CrRF), BCGF-II 30-60 I~CJrid0ma gn~vth factor (HGF), BCDF, 26

iFN~z. 26K protein BCGF-I, BCGF ~ 12 (60) - 20 -- 25

Major species Principal Other targets (kDa) sources

Multiple Multiple

T cells T cells, m0n0cytes T cells T cells, mast cells Tcells Eosinophils Multiple Multiple

T cells Leuk0cytes Multiple T cells Multiple

B-cell actions Ref.

Growth (c~fact0r); differentiation (co factor)

Growth; differentiation 11 Activation, growth; differentiation 59 (Growth; differentiation) 60 Differentiation 61

,;rowth 3 Differentiation 62 Inhibition; growth; differentiation 12

34O

and J. Gordon, unpublished observation). Most, if not all, replication factors also appear to be able to promote Ig secretion during B-cell stimulation. On the other hand, some m~lecui~s nave been claimed to direct differentia- tion in the absence of growth promotion. This notion is now compromised, however, as a major species in this category, cloned and termed BSF-2, has been shown to be identical to hybridoma growth factor (HGF), also known as IFN-13z and 26K protein, thus suggesting a strong growth component for this particular differentia- tion factor 14.

~ k ~ d e ~ As with yeast cells, bacteria and fibroblasts, it has

Resting

T m 0

ENLARGEMENT aoo ~ aOA \ __ao.\

\\ ~ \ ~ ~CHROMATIN I [ ~ - - ' ~ ' - ' * ~ t J - - ~ - ' * ~ \ OECONO ~ v CO-23~ ~ ~ "

CELL \ ~. G,A ~,

"~ '*"~"AT~ - - ~ 9 & h ~ l 4 - - """ " DNA symh. ~__.... ,.~-=t

2"3 days \ 3 6 h Tf-R!

RNA synth.

Replication

Differentiation

4d.

5d.

1 1

Fig. 1. A model for B-cell growth and differen6ation. The recen~ identified subcompartments of GO (Ref. 18) are indicated together

with g-~ progression of B cells through GI into S and onto replication and differentiation. An approximate time.scale is indicated for these events in vitro along with their accompanying phenotype such as the decondensation of chromatJn, which occurs as cells enter the highest state of activation within GO.

become clear that B lymphocytes encounter points of decision beyond those represented by simple quiescence, mitosis and terminal differentiation. Thus, their growth cycle is punctuated by a series of potential arrests where the cells take stock of their internal and external situation before proceeding accordingly. The existence of G2 arrest for B cells has already been mentioned. Classically, however, the major points of control have been taken to reside in the G 1 phase of the cycle, after the cells have left GO but before they have initiated DNA synthesis in S phase (Fig. 1).

Progression from the quiescent state into and through G1 is characterized by cellular enlargement, RNA syn- thesis and the expression of new proteins, some of which appear at the cell surface. For murine B cells, the most dramatic feature of G1 entry is a huge increase in expression of the la antigen 15. Less marked increases in major histocompatibility complex class II expression have also been noted for human B lymphocytes as they leave GO (Ref. 16). More reliable markers for this stage of the cell cycle in the human are the acquisition of a broadly distributed activation antigen defined by the monocional antibody 4F2 and a corresponding loss of IgD from the cell surface 17. it takes approximately 24 hours for quies- cent cells to achieve this stage of activation and it is here that they become responsive to the action of progression factors. As cells move through G1, receptors for trans- ferrin appear, signifying the requirement of cells for transferrin-bound iron in order to initiate DNA synthesis 17.18. IL-1 may also be exerting its growth- promoting effect at this GI/S transition stage 19.

The assumption that cells out of the cycle reside at a homogeneous level of quiescence, with the first activa- tion state available being early G1, can no longer be held. Certain activators induce phenotypic and function- al change in resting B cells which never leave GO (Ref. 18). The attributes of the GO subcompartments recently identified for human B cells are outlined in Fig. 1. Of special interest is the appearance of the CD23 antigen on GO cells as early as 2-4 hours after activation. Although the data are incomplete, the only cytokine claimed to induce CD23 at the resting B-cell surface is IL-4 (Ref. 20). At least with murine B cells, the heightened state of GO which is reached with IL-4 is accompanied by a more rapid entry of cells into S phase on their subsequent exposure to anti-lg 7. Thus, it is envisaged that during an immune response, one effect of this particular T-cell product would be to recruit B cells to respond swiftly to antigen challenge.

Immunology Today, vol. 8, No. 1 ~, ~987 @

Receptors and factors The appearance, de novo, of antigens such as CD23 at

potential points of control within the B-cell programme suggests that these molecules may serve as foci for the external signals which would then guide the cells to the next phase of the response. Similarly, antigens already present on the resting B cell could act as receptors for factors or cellular interactions which either initiate, potentiate or negatively control primary stimuli. The scheme represented in Fig. 2 attempts a synthesis of the cytokines and surface molecules which have been ascribed a functional role in human B-cell physiology. It is not intended as dogma, but instead aims to provide a framework in which the complexity of the events and interactions occurring at B-cell surfaces can be inte- grated.

The best characterized of the initial triggers is un- doubtedly that focused through surface Ig (smlg) either by antigen or antibody. The negative signalling through Fc receptors for IgG (Fc~/) observed when using whole anti-lg probably represents in-vivo feedback through immune complexes and, possibly, anti-idiotype, it was recently found that IL-4 interrupts this negative feedback by allowing B-cell triggering to proceed when the Fc receptor is occupied 21. Physiologically, such a mechan- ism would ensure that B cells were not inappropriately paralysed by immune complexes in the face of continued free antigen challenge. Further evidence for a strict homeostasis operating at these early stages of activation comes from the observation that IFN-~/ negates the stimulatory influences of IL-4 on resting B cells 2. At least for human splenic B cells, however, IFN-~/has also been described as a priming factor in a manner analogous to that of 11.-4 in the murine system z2.

A primary stimulus which may represent an alternative pathway to antigen triggering is that delivered through the CD20 molecule 23, while additional negative signal- ling for antigen stimulations may be operating through CD19 (Ref. 24). Both the CD2! (C3d receptor, CR2)and CD22 molecules provide signals that potentiate B-cell responses 2s.26. CD21 is of particular interest as it repre- sents the route of entry of Epstein-Barr virus (EBV) into the B cell. Antibodies binding at or close to the virus- binding region of CD21 are capable of providing a T-cell dependent stimulation to B cells 25. These effects deliv- ered to human B cells through CD21 are compatible with those of C3d at early stages in the murine B-cell cycle 4. Further possible cross-talk between the complement system and B lymphocytes has been suggested by the observation that the Ba and Bb components of alterna- tive pathway factor B are, respectively, inhibitory and stimulatory for B-cell responses 27,28.

A surface molecule which delivers a particularly powerful co-stimulating activity for primary signals is the recently identified CDw40 antigen. The signal transmit- ted through this 50kDa protein needs to be delivered concomitantly or shortly after the initial stimulus to act maximally 29. We have recently found that quiescent B cells can be perturbed directly through this molecule to enlarge, aggregate and remain responsive to mitogenic triggering (unpublished observations). Furthermore, in restimulation assays, antibodies to CDw40 exhibit strik- ing synergy with other growth-promoting activities for sustaining the proliferative cycle (unpublished observa- tions). While CDw40 is still a receptor in search of a ligand, its potential importance in B-cell physiology

should not be underestimated.

Activation Progress ion Repl icat ion Differentiation

Potentiation

C3cl CD22

CDw40

s°,° I I IL4 ~ II.-1 CD20 CD23 l 21 L21 IFN-3' --~ BSF-2

BCGF~j. IFN-cz

Inhibition

Fig. 2. Receptors and factors for human B cells. Some of the major actions which have been described for surface antigens and factors in

B-cell growth and differentiation are indicated (see text for details).

The provision of progression signals by 12 kDa BCGF and IL-1 and the replication activity of IL-2, IFN-~/and high molecular weight BCGF (which may or may not be IL-5) have been discussed already, while the pivotal role of CD23 in these processes will be dealt with shortly. Molecu!es controlling differentiation include BSF-2, 11.-2 and IFN-~. Factors which may direct the preferential production of individual Ig isotypes have also been described, particularly in the routine system. It is not yet clear at what level such activities might operate (switch, selection, enhancement) but it is known that the IgG 1- inducing factor is identical to IL-4, which in turn can also influence an antibody response in favour of IgE production3O. 31 .

Autocrine considerations Until now, this discussion has centred firmly on the

signals received by the B cell as if it were merely a passive acceptor of external influences, waiting to be steered tc its ultimate destiny as an antibody factory. Evidence is mounting, however, that the B cell itself provides regula- tow, influences in the immune system over and above that of Ig prcdu~ion, Fo!lnwing an early report that certain B-cell lines produced factors which potentiated normal lymphocyte activation 32, it was subsequently shown that some cell lines exploited these products in autocrine modes of growth enhancement 33. Further- more, for B cells which had been immortalized by EBV, their proliferation was strictly dependent on the con- tinued production of autostimulatory activity, of which a component, at least, had properties consistent with a progression factor and thus resembled a BCGF-134.35. It later transpired that autocrine growth stimulation was not restricted to the transformed state but could also accompany certain T-cell independent stimulations of normal B cells such as that achieved with the polyclonal activator SAC. In both situations, the BCGF activity was confined to a molecule of 25-35 kDa (Ref. 36). Other activities, most notably IL-1 and IL-1 inhibitors, have also been described 37. The properties of B-cell-derived mole- cules which have IL-1 activi~ appear heterogeneous. At least for one B-cell line, the IL-1 produced represents a 341

0 Immunology Today, voL 8, NO. 11, 1981

342

unique molecule, distinct from both IL-I~= and IL-11~ at its amino terminus; it lacks endogenous pyrogen activity but carries all the same biological information as monocyte-derived I!.-1 for in-vitro assays 38, It is clear, however, that some EBV-transformed lines express II.-1 genes conventionally associated with cells of the mono- cytic lineage 39. There is evidence to suggest that EBV- transformed B cells use their BCGF-like and IL-l-like activities synergistically to maintain autocrine growth 4°. We and others have now shown that, in addition to the soluble factor involvement, cell-contacts, or cell homo- typic adhesions are also a requirement for T-cell indepen- dent B-cell growth in both normal and transformed states 41.~. The nature of the signal conveyed through these intercellular bindings is unknown. Some reports suggest that the lymphocyte function antigen LFA-1 is involved in these cell contact phenomena, while membrane-bound IL-1 activities have also been implicatecH3. ~.

Whereas transformed B cells retain an autocrine phenotype, for normal B cells autostimulation is sus- tained over one or .No rounds of division at most. The rationale for normal autocrine processes is presumably to allow rapid expansion of proliferating clones in areas of high B-cell density. Similarly, to ensure control, such a mechanism would need to be self-limiting. The dys- regulation of autocrine growth accompanying trans- formation could reflect an inappropriately maintained production of factor, an inability to down-regulate func- tional receptors or a composite effect of the two. Recent reports have implicated a single molecule - the CD23 antigen - as central to all these considerations.

1 ~ Cl)~ mol~ule Not only is CD23 (also known as BLAST-2) abundantly

expressed on EBV-transformed B cells but its expression is also a prerequisite for transformation to proceed 45. The suggested link between CD23 and growth was confirmed recently by studies showing that the CD23 antibody MHM6 could replace 12kDa BCGF as a progres- sion factor for tonsillar B cells which had been activated by the phorbol ester TPA 19. The CD23 antibody, like BCGF, acted optimally 24 hours after the initial stimulus, so that cells in G1 entered S phase and progressed to the G2 phase of the cell cycle, where they arrested without proceeding to divide. IL-1, in natural or recombinant ~ or I~ form, augmented these CD23 stimulations by promot- ing the entry of cells in late G 1 into S phase (Ref. 19 and unpublished observations). Further progression of the G2-arrested cells required the addition of IL-2, which was presumably reflected on the increased level of Tac expression in cells stimulated via CD23 (Ref. 46).

Both the stimulating CD23 antibody and the 12kDa BCGF down-regulated the CD23 antigen, while other mitogens up-regulated the molecule as cells progressed into cycle. This specific effect of BCGF on CD23 expres- sion, coupled with the ability of CD23 antibodies to block the uptake of BCGF activity on to B cells, suggested some association between CD23 and the receptor for the T-cell-derived lymphokine. Lending support to this notion, we have recently found that a major conse- quence of the binding of stimulating CD23 antibody to its 45kDa target is the release of a 35kDa form of the antigen into the extracellular medium and this effect is mimicked by the binding of the BCGF, but not of other stimulating molecules, to the activated B-cell surface 47.

This is of particular interest as CD23 recovered from the supernatants of EBV-transformed B cells has autocrine growth-promoting activity 48. In the absence of a strin- gent cocktail of proteolytic inhibitors, the released 35kDa form of CD23 is rapidly processed to a 25kDa species and we now have reason to believe that the B-cell- derived BCGF discussed earlier, and the cleaved CD23 molecule are, in fact, one and the same. It then follows that a major effect of the T-cell-derived BCGF is to induce an autocrine growth factor cascade through its ability to process cell-bound CD23 to its autostimulatory ex- tracellular form. The mechanism of receptor cleavage is unclear, although its induction by univalent Fab frag- ments of CD23 antibody in an epitope-restricted fashion is suggestive of an allosteric process possibly similar to the generation of autoproteolytic activity recently de- scribed for the estradiol receptor 49. While the BCGF may work in a similar way, another possibii~ty is that the factor itself is endowed with a protease activity, as was recently described for certain other T-cell-derived effec- tor proteins s°. Thus, by binding at or near a sensitive site on CD23, the BCGF would bring about the proteolytic cleavage observed.

Whatever the mechanism, it is becoming clear that such processing of a surface molecule to a mature released form acting as a growth factor is by no means unique to CD23. Transforming growth factor-oL, epi- dermal growth factor, macrophage-colony stimulating factor (CSF-1) and, possibly, IL-1 are cytokines which appear to start life as surface-bound molecules. Whether these molecules also have corresponding ligands when surface-bound is unknown, although the possibility has been raised 51.

While most lymphocyte antigens are still in search of a function, CD23 appears to have an abundance of them. Several groups recently ~eported the cloning of the B lymphocyte IgE receptor52-54; one showed that the 45kDa product expressed was identical to CD23 (Ref. 55). With an affinity_ for IgE several orders of magnitude lower than the extremely high-affinity IgE receptor of mast cells and basophils, this quite distinct molecule is known to be expressed not only on B cells, but also on monocytes, hypodense eosinophils and a subpopulation of platelets. In the case of the last two, the receptor is involved in IgE-dependent killing 56. Interestingly, on platelets, the IgE receptor (i.e. CD23) seems to be expressed in association with GPIIb-Illa, a member of the LFA-1 family of molecules sT.

For B cells, a role for CD23 as an IgE receptor has been somewhat enigmatic. Partly in an attempt to reconcile its growth-promoting properties with its presumed IgE re- ceptor function, we recently examined the influence of IgE, agonistic antibody and low molecular weight BCGF on CD23 expression and B-cell stimuiation. The outcome was that IgE augmented both the BCGF and the anti- body stimulations and it appeared to do this by up- regulating CD23 expression, with the consequence that there was now more available receptor on which these agents could act to generate more cleaved receptor and thus provide greater autocrine growth-enhancing activity 47. A summary of this in-vitro model is shown in Fig. 3, together with the presumed involvement of other defined activators in the pathway. The implication is that direct T-cell independent mitogens, such as EBV and SAC, exert their influence, at least in part, through inducing the cleavage of CD23 constitutively, possibly via

Immunology Today, vol. 8, No. I 1, 1987 ~ .. ~ ~ ~ ~ ~ ~

in vitro

IL-4 ~ anti-lo ~ )

SACIEBV F~_~,. I

TPA BCG '~ ID i G' I

CD23

in vivo ?

BCGF m, <7 IL-4 !

¢ ~ 3 ',

Ag ~---J IgE

BCGF.ieh IL-4

Fig. 3. A pivotal role for CD23 in B-cell responses. (A) Model based on in-vitro observations (see text). (B) A possible in-vivo model for the interaction of growth factors, IgE and the

CD23 molecule in B-cell stimulation (see text).

the activation of an endogenous protease. In contrast, IL-4, which is claimed to induce CD23 on resting B cells, requires a co-stimulant such as anti-lg to activate the cleavage pathway. A possible in-v ivo relevance for the involvement of IgE in the CD23 cascade is also sug- gested. The essential feature here is the creation of a feedback regulation loop once an IgE response has been established. The large amount of free IgE required to demonstrate this circuit in vitro suggests that such conditions would be met physiologically only when IgE was bound to antigen. An alternative - or additional - role for feedback might be to militate against unaccept- ably high local levels of IgE by diverting B-cell activation away from IgE production towards other Ig isotypes. In this regard, it is worth noting that CD23 expression is confined to IgM-bearing B cells and, having switched, cells not only stop expressing CD23 but also lose their capacity to do so 58.

Conduding remarks The brief tour through the cell cycle, surface molecules

and growth factors of B cells begins to hint at where the

controls in the system might reside. Clearly, the numer- ous restriction points identified, coupled with the large number of factors impinging on them, imply that the detailed interplay of several activities working in defined sequence would provide a substantial proportion of the regulation required. It is important to remember that the in -v i t ro systems from which our knowledge has been gathered have been designed to test what is possible rather than what is necessarily occurring physiologically. Most, if not all, lymphokines appear to have an extremely short half-life in vivo so that the ability of B cells tO receive sufficient factor to respond will rely heavily on its interaction with the correct regulatory elements at the appropriate time. These influences, in turn, will be governed by such considerations as antigen localization, antigen concentration and the equally detailed regula- tion of the T-cell component of the response. With these considerations in mind, it is worth noting that while molecules such as 11.-4 are highly pleiotropic in their actions, the response they elicit may be tightly depen- dent, not only on the state of responsiveness of the B cell, but on the effective concentration of the factor in the local environment. For example, if we take the relative concentration of IL-4 required for optimal anti-lg co-stimulation as unity, then the amount reported neces- san/for activating resting B cells to CD23 induction is about 10%, while its IgGl-inducing activity and its IgE-enhancing effects have been reported, at least in. the mouse, as requiring approximately 10 times and 100 times the concentration, respectively 2°.3°.31. It becomes an attractive proposition that as an immune response progresses, the amount of favor required for each successive stage increases and this would be supplied only as long as the appropriate cells were still being signalled to produce. In this scenario, homeostasis will be provided ultimately by the presence (or absence) of antigen, the very reason for alerting the system in the first place. In individuals where that system has been • I , . ' ~ 1 " " ~, ,~ ,=u or maintained inappropriately vuw" .... in the case of immunodeficiency, high in the case of malignancy, aberrantly in the case of autoimmunity) the molecular description of the signals involved means that now, for the first time, there is high promise of a rational approach to therapeutic intervention to restore the normally finely tuned balance.

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Teaching aids from the centre pages of Immunology Today... r~ 03 Loci of mouse and human hbtommpatibility regions

Ref no. O# Cell ~ i n immune responses ~ . rxz OS ~ m e t ~ ~ of major H-2 haplotypes

no. 07 Surface I ~ and functions of rat Tlymphocytes no. 08 The bk~in-avidin c~rnplex in immunology no. 09 A new dasdfica~n of mouse V. sequences no. lo The sur~:3e phenotype of mouseTlymphocytes no. 11 l~el-lLA-Dmgion: genesand proteins

Ref no. 12 Mechanisms of autoimmunity in spontaneous thyroiditis Ref. no. 13 The evolution of MHC dass I genes Ref. no. 14 The evolution of MHC dass II genes Ref. no. 15 Computer models of the human immunoglobulins and T-cell

antigen receptor (with stereo viewer) I. Shapes and structures II. Interacting molecules

Ref. no. 16 Sequence patterns in domains of the immunoglobulin superfamily

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