230 immunology 1o@, December 7(.)81

) Heavy chain synthesis in pre-B cells

Since its identification by Raft and colleagues in 19761 the pre-B cell has successfully eluded strict charac- terization. The earliest recognizable cells of the B lineage were at first believed to be synthesizing com- plete IgM at a stage prior to its insertion in the surface membrane but it is now generally accepted that they produce g chains in the absence of light chain 2, almost to the point where this property has become a working definition for pre-B cells. Nevertheless, the precise role and biosynthetic pathways of pre-B g chain are still largely unresolved.

Controversy over the pre-B phenotype is un- doubtedly due in large part to the diversity of cell types being studied. Few workers have adopted normal fetal liver or adult bone marrow for study, probably because the frequency of pre-B cells present is low. Hybr idoma technology has enabled pre-B Ig synthesis to be amplified by fusion of mouse fetal liver cells with myeloma cells and has provided some interesting data 3,4. For strict biochemical analysis, however, established cell lines with pre-g-l ike characteristics have proved the most profitable for study: of particular interest has been 7OZ/3, a line adapted from a chemically induced tumour of BDF t mice.

Paige, Kincade and Ralph showed that on exposure to the B-cell mitogen lipopolysaccharide (LPS), 7OZ/3 cells which previously synthesized immuno- globulin detectable only intracellularly, rapidly acquired surface membrane Ig 5. Intriguingly, the induction of surface Ig was accompanied by the appearance of light chains 6. Perry and Kelley observed that Ig surface expression on 7OZ/3 cells was accom- panied by a ten-fold increase in the level of light chain m R N A but not of ~t chain m R N A 7. Taken together these results suggested that the onset of light chain production might be a prerequisite for surface Ig expression. However, several recent reports show this not to be the case.

With the same 7OZ/3 line but this time stimulated with dextran sulphate, Paige and colleagues have now demonstrated the acquisition of ~t chain in the absence of detectable light chain on the surface of these cells a. The surface g chain clearly has some unusual properties. Cells stained with affinity-purified anti- bodies displayed tight caps which were partly inhib- ited by prior treatment with cytochalasin B but unaffected by azide. To what extent these observations relate to normal B-cell physiology is unclear. The work of Levitt and Cooper with normal mouse fetal

liver cells suggests that pre-B g chains are capable of secretion but gives no evidence for a surface phase 9. Nevertheless, some recent studies on a sub-group of human acute lymphoblastic leukaemia 1°,1~ support the argument that the expression of sparse amounts of isolated surface g chain may represent a stage in normal differentiation corresponding to a transition from pre-B to B lymphocytes. However, pre-B char- acteristics ought not to be ascribed to cells solely on the basis of isolated heavy chain expression.

We have recently described a h u m a n B-cell lymphoma where the patient 's tumour cells displayed surface g chains in the absence of any detectable light chain ~2. These g chains migrated on SDS-gels as monomers with no evidence for inter-chain covalent linkage. The surface g chains rapidly capped but not to the extent described by Paige el al. for the dextran- sulphate-stimulated 7OZ/3 cells. In this case, how- ever, the lymphoma clearly had the tollicular centre cell histology usually associated with mature B lymphocytes. In these cells, then, the 'g-chain-only' phenotype was probably indicative of neoplastic change rather than a pre-B origin.

The role of pre-B g chain, whether secreted or surface-expressed, remains speculative. The asyn- chronous onset of heavy and light chain synthesis may be involved in the expression of antibody diversity, particularly as 'g only' hybridoma cells made with fetal liver cells have been shown to possess un- rearranged light chain genes 4. It is an attractive proposition that the heavy chains may interact with the external environment through their variable regions, influencing events such as clonal abortion, tolerance induction and cellular involvement in the idiotypic network. On the other hand, isolated Ig chains express only a small p ropor t ion of an antibody's idiotype and it is difficult to see why a molecule which by itself does not define antigen specificity should be thus implicated. The existence of a pre-B line which can be induced to express either whole IgM or g chain only is likely to be of consider- able value in approaching these fundamental ques- tions.

J O H N G O R D O N

Lympkoma Research Umt, Tenovus Laborato U, Southampton S09 4XY U.K.

References 1 Raff~ M. C., Megson, M., Owen, J. J. T. and Cooper, M. D.

(1976) Nature (London) 259, 224-226

© Elsevier/North-Holland Biomedical Press 1981 0167 -4919/81/0000 0000/$2,75

immunology today, December/981 231

2 Burrows, M., Lejeune, M. and Kearney, J. F. (1979) 2¢ature (London) 280, 838-841

3 Riley, S. C., Brock, E. J. and Kuehl, W. M. (1981) Nature (London) 289, 804-806

4 MaLl, R., Kearney, J., Paige, C. j. and Tonegawa, S. (1980) &ience 209, 1366-1369

5 Paige, C. J., Kincade, P. W. and Ralph, P. (1978)J. Immunol. 121,641-647

6 Ralph, P., Paige, C. J. and Nakoinz, I. (1979) in T and B Lymphocytes: Recognition and Function (Bach, F. H., Bonavida, B., Vitetta, E. S. and Fox, C. F., eds) 143-154, Academic, New York.

7 Perry, R. P. and Kelly, D. E. (1979) Ceil 18, 1133-1139 8 Paige, C. J., Kincade, P. W. and Ralph, P. (1981) Nature

(London) 292, 631-633 9 Levitt, D. and Cooper, M. D. (1980 Cell 19, 617-625

10 Brouet, J. c., Preud 'Homme, J. L., Penit, C., Valensi, F., Rouget, P. and Seligmann, M. (1979) Blood 54, 269-273

11 Vogler, L. B., Preud 'Homme, J. L., Seligmann, M., Gathings, W. E., Crist, W. M., Cooper, M. D. and Bollum, F.J. (1981) Nature (London) 290, 339-341

12 Gordon, J., Hamblin, T.J., Smith, J. L., Stevenson, F. K. and Stevenson, G. T. (1981) Blood 58, 552

T-B co-operation examined by 'counterfeit' systems using cell surface antigens

Ser ious i m m u n o l o g y came into be ing with the discovery of alloreactivity and the impor tance of the major his tocompatibi l i ty complex ( M H C ) seemed to reside in the genetic basis of alloreactions. 12 It has turned out that the true function of the M H C is to guide regulatory and effector T lymphocytes 3. In evolutionary or physiological terms alloreactions are now deemed to be artefacts and, accordingly, experi- mental models of allogeneic interactions, judged by natura l laws, should be considered 'counterfei t ' systems. However, we continue to benefit from counterfeit systems, either directly, as in clinical trans- plantat ion, or indirectly, by ext rapola t ing information from allogeneic systems to fit physiological regulation. I intend here to review briefly some recent benefits derived from such systems in studies of the regulat ion of B-cell responses by helper T cells (Th cells).

In general T - B co-operat ion involving responses to cell surface antigens, can be examined best in vitro by assays of the 'positive allogeneic effect' type, where Th cells react directly with surface antigens on B cells 4s and in vivo by systems involving linked recognition of antigens on an allogeneic cell by both Th cells and B ceils, which in this case are syngeneic 6. These two types of reactions have been recently te rmed allo- specific-alloregulatory and allospecific-synregulatory, respectively 7. The impor tant question is whether these two systems (of which the first qualifies as counter- feit) measure the same type of regulatory activity. We have recently obta ined evidence in our labora tory that this is indeed the case. Clones of Th cells directed at allogeneic I-A products were active to comparable degrees in both systems (Yeh Mind et al., unpub- lished). This strengthens our confidence that counter- feit systems of the allogeneic type measure events identical to, or at least closely resembling normal regulation.

Can such interactions involving M H C products be dupl icated in systems where Th cells are directed at other cells' surface molecules? It is clear from at least three recent studies using homogeneous Th cell pop- u ta t ions tha t in te rac t ions of this k ind can be successfully examined by directing Th ceils at minor his tocompat ibi l i ty antigens on B cells. Two of the

papers in question were presented at the 14th Inter- nat ional Leucocyte Culture Conference in Heidelberg in June, 1981. Lai et al. s mainta ined Th cells specific for mult iple foreign minor his tocompatibi l i ty antigens in long-term culture, and demonst ra ted that these retained specific helper activity for B cells expressing the original s t imulat ing antigens for an ant ibody response to sheep red blood cells (SRBC). Moreover, the same homogeneous Th cell populat ions were active in vivo in an adoptive transfer system of the linked recognition variety ~, confirming that the T - B interaction measured in vitro resembles physiological help. Pettersson et al. (Pettersson, personal com- municat ion) examined Th cell lines and clones directed at the male antigen H-Y and found that these could co-operate with male but not female B cells for immunoglobul in ( IgM and IgG) product ion read by a protein A plaque assay. This col laborat ion was M H C restricted. Similar findings to Pettersson's were made recently by R. Zubler (Zubler, personal communica- tion) in a system where B-cell IgM and IgG produc- tion is read by ant i - t r in i t rophenyl or ant i -SRBC plaques. The use of a male-specific killer T-cell clone for plaque typing elegantly demonstrates that in a mixture of male and female B cells, co-cultured with ant i-male Th clones, all co-operat ing B cells are male. This indicates that direct recognition by Th cells of surface antigens on B cells is a requirement for co- operative interact ion and that ' bys t ande r ' effects are not operative.

In a way these experiments are also counterfeits by physiological s tandards since lymphoid cells ot" female and male phenotypes are asked to co-operate - unlikely circumstances except in aberrat ions like mosaic hermaphrodi t i sm. However, such in-vitro interactions have the advantage of simplicity and can be verified in physiological systems. Ant i -H-Y clones have not yet been examined for help in vivo, but this should now be possible in Mitchison 's system where male-specific Th cells provide intrastructural , inter- molecular help to syngeneic female B cells responding to Thy 1 (Mitchison, unpublished).

Most of the experiments discussed sQ far were done with homogeneous populat ions of Th cells. This is

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