Paradigm regained

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symptoms (J. H. Hoofnagle, NIADDK, Bethesda). Of considerable concern is the high frequency of persistence of liver disease in patients with chronic non-A, non-B hepatitis. In one stud),, 68 % of cases showed continuing signs of liver damage one year after onset of disease many with chronic active hepatitis progressing to cirrhosis (H. J. Alter, Clinical Center, Bethesda). All attempts to define a serological marker for non-A, non-B hepatitis have proved unsuccessful; despite over 30 reports in the literature identifying specific antigens, multicentre studies co-ordinated by H. Alter of the Clinical Center, National Institute of Health have shown conclusivety that none of these are specific. A

collaborative study among various groups at N I H have found that a rheumatoid fhctor-like IgM is frequently present in acute phase, chronic non-A, non-B sera which reacts with antibody in various categories of"multiply transfused individuals. This IgM molecule is not demonstrated by" conventional rheumatoid factor tests but is adsorbed by aggregated IgG and is destroyed on reduction (S. M. Feinstone, NIAID, Bethesda), and this may account for the claims from various laboratories that they have developed a test for a specific antigen. Certainly one of the major difficulties in the study of non-A, non-B hepatitis is the apparently low titre of infectivity in most units of contaminated blood. Although

The T-cell antigen receptor

Paradigm regained from Jonathan Howard About 18 months ago, Jensenius and Williams reviewed the state of knowledge about the T-cell receptor, and summarized their views as follows: 'It has been dogma that the antigen receptors on T lymphocytes must have recognition sites that are essentially the same as those of antibodies and that the receptors would be found if structures related to immunoglobulin (Ig) V-domains could be identified on T lymphocytes. In a critical assessment we conclude that there is no sound basis for these views.'1. The extraordinary torrent of information that has been pouring out of North America since then has shown how very completely wrong Jensenius and Williams were. It is true that the methods used to solve the problem were novel. The solution turns out to be so Ig-like that some of my colleagues have declared themselves to be disappointed. The culmination of this process of restoring the immunoglobulin paradigm to its place comes with the recent publication of the complete primary sequence of both the alpha and beta chains of the putative receptor of a mouse cytotoxic T-cell clone 2. Before discussing the implications of this latest advance, let us recapitulate briefly the chronology of this amazing story. At the end of 1982James Allison and colleagues at the University of Texas, Smithville, prepared a monoclonal antibody against a mouse T ceil lymphoma membrane marker. The antigenic determinant was 'idiotypic' in the sense that it was borne only by the immunizing lymphoma and not by other superficially similar cell lines: a tumour-specific antigen, in other words. The molecule identified on the © 1984, Elsevier Science Publishers B.V,, Amsterdam

cell surface was a disulphide-bridged heterodimer, the two chains both having apparent relati;ce molecular weights on SDS gels in the region of 40 000. Because of the disulphide bridge, the molecule ran off the diagonal on two-dimensional SDS gels in which the first dimension was unreduced, and the second reduced. Allison and his colleagues were able to show that a molecule with this characteristic property was also carried on normal T cells, and serological crossreactivity betwen the lymphoma molecule and the normal counterpart was demonstrated by means of a rabbit antiserum raised against immunoprecipitated lymphoma material a. The possibility that this molecule might be related to the T cell receptor was explicit in the first paper from this group 4. Two other groups were also making antiidiotypic antibodies against T cells, but now against either antigen-specific mouse T cell hybrids or against antigenspecific human T cell clones. Monoctonal antibodies were developed which interfered with specific T cell function, and showed idiotypic specificity. In both cases the molecules identified on the cell surface were also disulphide-bridged heterodimers with two chains of around 40 000 relative molecular weight 5'6. The intimate association between this molecule and specific antigen recognition by T cells was shown by the important discovery that the idiotypic specificity could predict the fine specificity of both antigen and M H C recognition 7. Work at the protein level was rapidly foUowed by eDNA cloning. Mark Davis and colleagues s'9 looked for T cell

0167 - 4919/841rd)2,00

chimpanzee transmission studies clearly show that there are at least two distinct agents involved, no definite isolation of virus has been reported. At least one form may be an enveloped virus judging by its sensitivity to lipid solvents with a diameter less than 80 nm (D. A. Bradley, Center for Disease Control, Atlanta), although studies of faecal material collected from a recent epidemic non-A, non-B in Nepal suggests that a hepatitis A virus-like particle 27 nm in diameter may also be implicated (M. A. Kane, Centers for Disease Control, Atlanta).

C. R. Itoward is in the School of Hygiene and Tropical Medicine, London W1, UK.

specific messages whose genomic sequences were rearranged in mature T cells. For this elegant approach to work it was necessary both that the T-cell receptor message should not hybridize significantly with Ig message, and that the T-cell receptor genes should rearrange during T-cell ontogeny in a manner for which only immunoglobulin genes provide a precedent. In other words, this experiment assumed the falsity of the immunoglobulin paradigm for the purpose of sequence, while taking the same paradigm for granted for the purpose of rearrangement. In the event this schizophrenic approach was rewarded and the first sequence of a mouse T cell receptor beta chain was obtained. It was immediately apparent how very' immunoglobulin-like this sequence was in overall organization 9. Not only was the molecule built from two disulphide-bridged domains of exactly the same size and general structm-e as Ig domains, but there was strong amino acid sequence homology at several critical sites adjacent to the intra chain bridges. It was also clear that the N-terminal domain was variable, and the C-terminal constant; there was evidence for independent J and more recenfly D segments between the two domains. To an extraordinary degree the beta chain of the mouse T cell receptor resembled the first two domains of an immunoglobulin chain, with the addition of a hydrophobic transmembrane segment and a short intracytoplasmic tail. The m R N A for the/3chain of a human T-cell receptor was isolated from a T-cell line and identified by sequence homology with Ig1°. More recendy the disposition of V, J and C genes of the mouse and human T cell receptor beta chain have been investigated in the germ-line configuration. An upstream V family is followed at an unknown distance by a tandem pair of

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J-C clusters. Thus each of two C genes is preceded by a set of J sequences. It is clear that both the C genes are beta chain genes. The arrangement is reminiscent of mouse lambda n 13. Not only is the arrangemenf of gene segments in the germ-line entirely familiar from previous experience with the Ig genes, but even the pattern of amino acid substitutions inferred from the few V sequences compared suggest the functional division of the V domain into hypervariable and framework sequences corresponding in position to the equivalent sequences in Ig V regions. From sequence homology it was clear that only the beta chain of either h u m a n or mouse T cell receptor had been found. Now the alpha chain has dropped out of a T-cell specific message library of a mouse alloreactive cytotoxic clone, along with another beta chain. At last it seems as if the complete primary structure of a T-cell receptor is in our hands. So what does it look like? Exactly as predicted by Alan Williams in a recent commentary on the publication of the first beta chain sequence 14, the alpha chain is a virtual replica of the beta chain. It has all the features of immunoglobulin structure that were so striking about the beta chain, with evidence of V, J and C regions, two disulphide-bridged domains, a transmembrane hydrophobic segment and a cytoplasmic tail. An unpaired Cys at residue 234 in the alpha and 236 in the beta chains presumably is responsible for bridging the two chains. As with the r-chains, the a-chain genes are re-arranged in T-cell lines. The implications of this remarkable structure for the T cell receptor are of fundamental importance for immunology. T h e urgency with which the hunt for the T-cell receptor has been pursued is not due simply to the fact that the receptor has been elusive, but also, and most importantly, because the specificity of T cell recognition has raised major theoretical problems. T-cell specificity is usually complex, with nominated antigen apparently co-recognized with cell surface major histocompatibility complex ( M H C ) molecules. H o w is this dual recognition achieved? Because the solution turns out to be simple, we can afford to be simplistic in describing the

great debate between one and two receptor models for MHC-restricted T cell recognition. The fact is that the structure of the T-cell receptor that can be inferred from the results of Saito et al. 2 is just about the only one it could have had which could lay the old debate to rest with sequence None. The structure of the T-cell receptor is so much like that of an Ig that its function can be interpreted in the same terms. W e can be confident that the two variable domains, one from each chain, interact with each other so that hypervariable residues from the two chains are brought into close apposition, forming a pocket or cleft of unpredictable structure whose affinity for antigen is a consequence of contributions from both chains. Such an interactive site gives no comfort to those who have accepted that the T cell receptor is a single molecule but who retain a belief in independent affinity of subsites for nominated antigen and M H C , the crypto-two-receptor model perhaps most familiar from the writings of Schwartz and his colleagues 15. If the T cell receptor really does look so much like an immunoglobulin, we should really go back to the most uncompromising of the onereceptor models 16 and start again from there. Two more points are of interest. Firstly, it looks as if the T-cell receptor is a monomer, that is, two chains form a single site, and each such site is independently anchored in the membrane. I have never been quite clear what advantage accrued to the m e m b r a n e Ig receptor of B cells from being a dimer: was it simply that the ultimate secreted product was a dimer, and the membrane-secreted alternative splice site was C-terminal of the inter-chain bridges? Secondly, the beta chain constant region sequenced by Saito et al. 2 is clearly the same one sequenced by Davis and his colleagues. Since Saito used a cytotoxic clone, and Davis a helper, it is clear that at least the beta chains show no isotype variation associated with function. If the alpha chain follows the same pattern, and this is a big if, then we shall have to conclude that the characteristically different functions of these cell types are due to properties of the cells that are independent of their receptors. Furthermore

it seems likely that use of the same C-region genes will entail use of the same V-gene families. W e therefore face the near-certainty that the typical specificity preferences of killer cells for antigens associated with class-I M H C molecules and helper cells for antigens associated with class-II M H C molecules are due to some undefined selection process operating on a single repertoire. One must now anticipate that the correlation between T-cell function and specificity is likely to be guided by exogenous signals and it is difficult to see how the recognition of different classes of M H C molecules can fail to play a role in these signalling processes. It will be of enormous interest to know whether the final pathway of differentiation is in fact established before exogenous antigen begins to impinge on the developing lymphon, or whether recognition of endogenous M H C molecules alone provides the selective stimulus. [] . f C. Howard is in the Department of Immunology, A R C Institute of Animal Physiology, Babraharn, Cambridge.

~e~ercnces

l Jensenius, J. Cl. and Williams, A. F. (1982) Nature (London), 300, 583 2 Saito, H, Kranz, D. M., Takagaki, Y., Hayday, A. C., Eisen, H. N., and Tonegawa, S. (1984) Nature (London) (in press) 3 McIntyre, B. W. and Allison, J. P. (1983) Cell 34, 739 4 Allison,J. etal. (1982)J. Immunol. 129, 2293 5 Meuer, S. C. et aL (1983)J. Exp. Mad. 157, 705 6 Itaskins, K. et al. (t983)J. Exp. Med. 157, 1149 7 Marrack, P. et al. (1983)J. Exp. Mad. 158, 1635 8 Hedriek, S. M. et al. (1984a) Nature (London) 308, 149 9 Hedrick, S. M. et aL (1984b)Nature(London) 308, 153 10 Yanagi, Y. et al. (1984) Nature (London) 308, 145 11 Chien, el al. (1984) Nature (Londor0 309, 322 12 Siu, G. et al. (1984) Cell (in press) 13 Malissen, M. et al. (1984) Cell (in press) 14 Williams, A. F. (1984) Nature (London) 308, 108 15 Schwartz, R. H. (1984) in Fundamental Immunology (ed. W. Paul) p. 379, Raven Press, New York 16 Matzinger, P. (1981) Nature (London) 292, 497