EXPRESSION OF STRONG Mls DETERMINANTS IN

Although typical mixed lymphocyte reactions (MLR) 1 are directed predominantly to H-2 determinants , high pr imary MLR in mice can also be elicited by gene products o f the minor lymphocyte-stimulating (Mls) locus located on chromosome 1 (1). O f the four described Mls alleles, a b c d, Mis a and Mls d determinants elicit the strongest response. Since these two sets of determinants cross-react extensively (and are probably indistinguishable), we shall re fer to them as "Mls a'd determinants" (2). Mis a'd de terminants are o f interest for at least three reasons. Firstly, in our hands the T cell response to these determinants does not exhibit H-2 restriction (3, 4) (although others disagree with this viewpoint [5]). Secondly, f rom studies on karyotypically unstable hybridomas prepared f rom a dual-reactive T cell clone, we have prel iminary evidence that the receptors for allo-H-2 and Mls a'd determinants are encoded on dif ferent chromosomes. 2 Thirdly , the expression of MIs a'd determinants appears to define one of the two subsets of B cells, namely the "mature" B cells, which develop late in ontogeny and express Lyb 5 surface alloantigens (6); immature (Lyb 5-) B cells do not express Mls a'd determinants. Mls a'd determinants are thus a potentially useful marker for probing the functional differences between mature and immature B cells, and for addressing the key issue of whether these two subsets of cells represent distinct lineages or different stages of differentiat ion o f the same lineage. Much of the evidence on the differences between mature and immature B cells has come f rom studies on B cell development in CBA/N mice, i.e., mice expressing X-linked immunodefic iency (xid) (6). These mice contain immature This work was supported by National Science Foundation grant 8303042 and National Institutes of Health grants CA-15822, CA-33958, and AI-10961. The present address of S. R. W. and J. s. is Dept. of Immunology, Scripps Clinic and Research Foundation, La Jolla, CA 92037. Abbreviations used in this paper: FCS, fetal calf serum; LN, lymph node; MLR, mixed lymphocyte reactions; MLS, minor lymphocyte-stimulating; PFC, plaque-forming cells; TDL, thoracic duct lymphocytes; TNP, trinitrophenyl; xid, x-linked immunodeficiency. 2 T cell hybridomas with dull reactivity for MIs ~'d and allo H-2 determinants can selectively lose one specificity. Webb, S. R.,J. Hu Li, I. MacNeil, P. Marrack,J. Sprent, and D. B. Wilson. Manuscript in preparation.