Immunology Today, Vol. 9, No. 5, 1988 ll.i

The MIs locus was described in the late 1960s by Hilliard Festenstein; cultures of lymphocytes with a disparity at this locus induces intense, unidirectional Tocell pro- liferative responses. The genetics and immunobiology of this immunological enigma were discussed at a recent Howard Hughes Medical Institute Work-

shop*.

The MIs locus was described nearly 20 years ago as a single gene trait that induced intense one-way mixed lymphocyte reactions (MLRs) be- tween cells of mice identical at the major histocompatibility complex (MHC). Since that time, it has re- mained essentially an enigma, with controversy surrounding the nature of the MIs gene product and the response generated in MLRs be- tween MIs-disparate strains.

The impetus to understand this locus stems from the fact that dis- parities at the MIs locus stimulate about 20% of T cells in primary mixed lymphocyte culture in a way that involves an interaction between the T-cell receptor and MHC class II molecules, leading to a breakdown in the usually precise recognition of these molecules. The mechanism by which this striking phenotype is achieved may have profound im- plications for the interaction of the T-cell receptor with MHC molecules. It may even elucidate that knottiest of problems, the developmental selection of the T-cell receptor reper- toire.

This workshop brought together individuals who have worked on this problem in recent years. The data presented helped to clarify some of the issues that have caused con- troversy in this field. The consensus view of this field is summarized be- low, and areas of continuing uncer- tainty or controversy are highlighted.

Genetics of MIs The first issue that needs clarifi-

cation is the genetics of the MIs trait. Recent work suggests that two seg- regating loci control the trait or traits originally called MIs by Festenstein. The most potent of these loci maps to chromosome 1, and encodes the classical MIs a trait. Lymphocytes from mice carrying MIs a are potent stimulators of T cells from mice

*Held at the Howard Hughes Medical Institute Con- ference Center, Coconut Grove, Florida, USA on 1--4 November 1987.

.... . e . 5

The Ms locus: new clues to a lingering .... mystery.

from Charles A. Janeway, Jr, Kirsten Fischer-Undahl and UIIrich Hammerling

carrying the only known alternative allele at this locus, MIs b. By contrast, cells from strains carrying MIs b do not significantly stimulate T cells from strains carrying MIs a. Excep- tions to this statement, presented by R. Click (Minnesota) and S. Kimura (London), were believed to represent responses to products of other minor histocompatibility loci, distinct from the potent primary responses of MIs b T cells to MIs a stimulator cells. From the work of R. Abe and R. Hodes (NIH), Click and J. Ryan (USUHS) it has become clear that the trait origi- nally called MIsc by Festenstein is determined by a segregating locus not linked to MIs a'b. Furthermore, the trait originally called MIs d by Festenstein appears to be MIs a plus MIsc expressed in the same strain, especially if that strain is H-2 k. Thus, the original MIs locus appears to be two independently segregating loci, of which MIs a on chromosome 1 is the most potent and of greatest immunobiological interest.

The conference grappled unsuc- cessfully with the question of nomenclature, and decided to retain the established nomenclature. The alternative, suggested by some par- ticipants to reflect the clear distinc- tion of the classical MIs a from MIs c, was to number the loci, and to designate alleles as responder (r or b) or stimulator (s or a) (Table 1).

M. Dorf (Harvard) reported that the genotype called MIs x by C. Jane- way (Yale) was actually an allele of classical MIs c, but assignment is made difficult by the role of the

MHC of the PL/J strain in which this trait was defined (see below). The unsettled question in the genetic analysis is whether there is any poly- morphism that cannot be accounted for by the presence or absence of an invariant product at a given locus. If MIs x and Misc turn out to be allelic at MIs-2, then allelism other than null allele versus productive allele will be established, and new complexity added to several models. This ques- tion is crucial to the various hypoth- eses for MIs biology (see below). In particular, polymorphism at a single locus is most easily explained by the MHC class II-binding peptide hypoth- esis.

The MHC and the T-cell receptor The second issue dealt with was

the role of the MHC in MIs recog- nition. Two questions have been raised: is the response of T cells to M/s-disparate stimulator cells MHC restricted in the classical sense, and do MHC class II antigens play any role in MIs responses? The answers suggest a consensus. First, for res- ponses of MIs b T cells to MIs a stimu- lator cells, little restriction by MHC haplotype is apparent. However, anti-class II MHC antibodies potently inhibit such responses - anti-I-E more so than anti-I-A. Second, cells from H-2q mice were weak stimula- tors in this situation, although F~ hybrids carrying MIs a from the H-2, strain and a different H-2 type were potently stimulatory. These data were generally agreed to impry that MHC class II molecules play an im-

Table 1. Proposed M/s nomenclature

Old nomenclature New nomenclature

MIs-I MIs-2

Typical strains

MIs a a b MIs b b b MIs c b a MIs d a a MIs x b a'

AKR/J B10.BR,C57BL/6

C3H/HeJ,BALB.K,A/J,BALB/c CBNJ,DBN2,C58/J

PIJJ 125

© 1988, Elsevier Publications, Cambridge 0167 - ,~919/88/$02.00

Immunology Today, Vol. 9, No. 5, 1988

126

portant role in this T-cell response. What is interesting about the MIs responses is the breakdown in the classically precise specificity for MHC class II alleles typical of MHC- restricted responses. Cloned T-cell lines specific for antigen and self I-A will respond to MIs a on a variety of different H-2 genotype stimulators, with I-E predominating in the response in many cases. Thus, although it is widely agreed that the stimulation of T-cell responses by MIs-disparate stimulator cells in- volves MHC class II, it is the loss of MHC class II specificity in such res- ponses that contributes much to 1;heir mystery and their fascination.

A central controversy about res- ponses to disparities at the MIs locus is whether these responses involve the conventional T-cell receptor complex, or whether, like cross- linking of some other structures, such as CD2 or Thy-1, they act inde- pendently of the T-cell receptor. Antibody blocking studies by O. Kanagawa and A. Glasebrook (Lilly, La Jolla) and by Janeway suggest that T-cell receptors and their MHC class II-specific component CD4 are involved in this response, while other experiments by S. Webb and J. Sprent (Scripps, La Jolla) raise doubts about the interpretation of such studies. Furthermore, hybridomas that appear to lose their antigen/ MHC class II receptor can retain reac- tivity to MIs (Webb; Sprent; J. Kap- pier, Denver and D. Wilson, La Jolla). A strong boost for those who have advocated involvement of the T-cell receptor in this response came from studies by Kappler. Kappler's group had noted that the T-cell receptor VI~ determinant detected by the mono- clonal antibody K J-16 was expressed weakly in strains that express MIs a. Using a second reagent, F23.2 from M. Bevan (Scripps, La Jolla), Kappler showed that the VI38.1 gene is not significantly expressed in strains that are MIs a, compared with strains that are MIs b. Furthermore, the level of expression of V138.1 in MIs a strains was governed by the MHC. Finally, Kappler isolated randomly selected T-cell hybrids expressing VI38.1 and showed that about 80% respond to MIs a. These cells show the same unrestricted responses to H-2- different, MIs a stimulators observed with other cloned T-cell lines. In con- trast, in the rarer cloned lines that bear V138.2 and are reactive to MIs a,

H-2 k stimulator cells do not respond to MIs a stimulators of other H-2 genotypes. These data strongly sup- port the earlier antibody blocking studies, and show by a completely independent technique that both the T-cell receptor and MHC are involved in responses to MIs- disparate stimulator cells. Compara- ble data on V~6 expression (not re- ported at this meeting) have been obtained by R. MacDonald and col- leagues (Nature, in press).

Thus, data from various ap- proaches support the concept that the response to MIs a stimulator cells by T cells from MIs b strains involves the interaction of the conventional T-cell receptor with MHC class II molecules. Furthermore, the dissec- tion of the original MIs trait into at least two separate loci simplified in- terpretation of many studies. What remained to be determined were the mechanism by which disparity at MIs loci caused such a remarkable T-cell proliferative response, and the bio- logical implications of this system.

Mechanism(s) of action Discussions of mechanism focused

on three ideas. First, MIs may encode a celi interaction molecule expressed at different levels in different strains which acts on a second molecule on the T-cell surface to promote activa- tion via the interaction of MHC class II with the T-cell recentar. This

i . . . . . . . . .

hypothesis is consistent with the apparent augmented antigen pres- entation by MIs a spleen cells re- ported earlier by Janeway and col- leagues. However, attempts to confirm this model have failed (Jane- way). A variant of this model sug- gests that the MIs locus product on the stimulating cell acts on a distinct receptor on the T cell which, in the new construction, would have to act in concert with the T-cell receptor for antigen. Preliminary data supporting this idea were presented by Webb.

A second hypothesis, proposed by Kappler, is that MIs encodes an MHC class II binding peptide that has very high avidity for many different T-cell receptor V regions, in particular Vl38.1, such that class II molecules associated with this product would stimulate cloned T-cell lines of widely varying specificity. Kappler sug- gested that peptides similar to that encoded by MIs might be selectively expressed in the thymus, associate with MHC class II during T-cell de-

velopment, and play a role in positive selection of T cells recognizing self MHC. The MIs locus could even rep- resent polymorphism in such a mol- ecule, or altered regulation of its expression, such that it is expressed on peripheral cells in MIs a strains, leading to direct stimulation in the periphery. This interesting idea is well worth exploring. R. de Kruyff (Stanford) has shown a two-cell be- havior pattern in MIs a stimulation, consistent with the existence of a peptide form of MIs. In addition, S. Buxser (Upjohn) and Janeway have been testing bacterial proteins that mimic MIs. Similar mitogenic pro- teins from Mycoplasma arthritidis have also been studied by Hodes and colleagues. These proteins, of de- fined structure, could lead by anal- ogy to the detection of molecular products of the MIs locus, which remains the biggest problem in this area.

Finally, U. Hammerling and M. Hoffman (Sloan-Kettering, New York) argue that MIs encodes a regu- latory molecule which acts to pre- vent autoreactivity. Polymorphism in this locus leads to uncontrolled auto- reactivity, which is detected as a mixed lymphocyte response. This idea is difficult to refute, as it ex- plains the data nicely. However, some argue that the F1 hybrid between MIs a and MIs b, which is st imulatnrv ~hnulrl art , J~llv h~ ¢tnmi_

nan t lyd own-regu la t-ory a'ccorcl ing 'i'o this model. In addition, Glasebrook has made a B-lymphoma hybrid that stimulates cloned T cells responsive to Mlsa; loss variants of this hybrid retain the ability to present antigen but no longer stimulate M/s-reactive T cells.

All the participants agreed that such models cannot be fully resolved unless the molecule(s) encoded by the MIs locus (loci)is (are)identified. Attempts in this direction have suc- ceeded in demonstrating that the Fc receptor or Lyl 7, while closely linked to MIs a on chromosome 1, is not the product ef this locus (B. Huber, Tufts; J. Ravetch, Sloan-Kettering), but have not yet gone further.

Charles Janeway, Jr, is in the Yale University School of Medicine, New Haven, CT 06510, USA; Kirsten Fischer-Lindahl is in the Howard Hughes Medical Institute, Dallas, TX 75235; UIIrich Hammerling is in the Sloan-Kettering Institute for Cancer Research, New York, NY 10021, USA,