Immunogenetics 30: 335-343, 1989 111llllllllO- gen e O'cs 9 Springer-Verlag 1989 Responses against antigens encoded by the 11-3 histocompatibility locus: antigens stimulating class I MHC- and class II MHC- restricted T cells are encoded by separate genes Derry C. Roopenian and Allan Peter Davis The Jackson Laboratory, Bar Harbor, ME 04609, USA Abstract. The purpose of this work was to study the genetic basis of histocompatibility antigens encoded by the mouse minor histocompatibility (H) locus H-3. Both class I major histocompatibility complex (MHC)-restricted cytotoxic T lymphocytes (CTL) and class II MHC- restricted helper T cells (TH) specific for antigens en- coded by genes within the H-3 locus were isolated and analyzed. Typing a number of mouse strains for expres- sion of antigens recognized by these TH and CTL sug- gested that there was a different strain distribution pattern of expression of the antigens recognized by TH compared with those recognized by CTL. Separation of the genes whose products stimulate TH from those whose products stimulate CTL was suggested by: (1) analysis of the strain B10.FS(92NX)/Grf that has undergone recombination within the H-3 region; (2) genetic segregation studies of (BlO.UW-H-3b/SnxC57BL/lOSn)F2 mice; and (3) F 1 complementation studies in which CTL specific for pro- ducts of the TH-defined gene(s) could not be detected, even in the absence of immune responses to products of the CTL-defined genes. Taken together, these data sug- gest that in addition to two genes (B2m and Cd-1) within the H-3 region whose products typically stimulate class I MHC-restricted CTL, there is at least one additional gene whose product selectively stimulates class II MHC- restricted TH. This new gene is located telomeric from the CTL-defined genes and between the loci we and un on chromosome 2. These data demonstrate a novel degree of complexity of the H-3 "locus" and suggest selective presentation of minor H gene products in the context of class I or class II MHC proteins. ing and selecting for rejection of tumor or skin grafts (Snell 1958, Snell and Bunker 1964). H-3 has been map- ped to the central portion of chromosome 2 approximately 18 centimorgans (cM) centromeric from agouti (a; Snell and Bunker 1964). Earlier reports suggested that this "locus" was complex and that the histoincompatibility at- tributed to it may actually be the result of the combined effects of more than one linked gene (Snell and Bunker 1964, Snell et al. 1967). These results were confirmed and extended by a number of investigators leading to evidence suggesting that the H-3 locus is comprised, in fact, of three or more linked H genes (Click et al. 1981, Rammensee and Klein 1983a, Graft et al. 1985, 1987). Two of these genes are known to produce products that act in an allele-specific manner as antigens recognized by class I major histocompafibility complex (MHC)-restrict- ed cytolytic T lymphocytes (CTL; Roopenian and Click 1980, Rammensee and Klein 1983a, Kurtz et al. 1985, Graft et al. 1985; see Table 1). One of these genes is thought to be beta-2 microglobulin (B2m; Kurtz et al. 1985, Rammensee et al. 1986); the molecular basis of the other gene is unknown. Although class I MHC-restricted CTL are commonly associated with minor H barriers, evidence also suggests that CD4 § class II MHC-restricted "helper" T cells (TH) appear to carry out a central role in immunity to minor H antigens (Loveland et al. 1981, Cobbold and Waldmann 1986, Rosenberg et al. 1987, Roopenian and Anderson 1988a, 1988b). In the current report we isolate TH specific for H-3 antigens and study whether they re- spond to products of the same genes to which the CTL respond. Introduction Materials and methods The mouse H-3 minor histocompatibility (H) locus was originally isolated on congenic strains through backcross- Address correspondence and offprint requests to: D.C. Roopenian. Mice. Miceused in these experimentswere bred and maintainedat The Jackson Laboratory. They were usually 2-6 months of age. Both male and femalemice were used. A descriptionof relevant genetics of some of the H-3-congenicstrains used in these studies is given in Table 1.