Reconstitution ofIrgenes, Iaantigens, andmixedlymphocyte reaction determinants bygenecomplementation

Byusing clones ofmurine Tcells reactive with alloantigens aswell assoluble antigens, ithasbeenpossible to demonstrate that Iaantigens, Irgenephenomena (defined asthe ability ofimmuneTcells torecognize antigen), andmixed lym- phocyte reaction (MLR)-stimulating determinants encoded within theI-Asubregion aredifferent manifestftions associated with sameproduct. These studies utilized theI-Amutant mouse B6.C- H-2bmlf (bml2), whose defeet inthenormal expression ofthecell surface products oftheI-A. subregion canbepartially circum- vented through trans-complementation byderiving hybrids be- tween bml2andmiceexpressing normal I-Asubregion products. Such micejbe.g., (bml2 xBIO.A)Fj) express several serologically normal I-Aproducts buthavedefects incertain I-Asubregion geneproducts normally expressed oncells ofH-2'X H-2bhet- erozygote micethat areused asrestricting elements forantigen recognition byantigen-reactive Tcell clones orrecognized asal- loantigens byalloreactive Tcell clones. This defect incell surface expression ofcertain normal I-Ab products on-the mutant bml2 cells hasallowed ustosugggst that there exist trans-complement- ing prodicts onH-2axH-2heterozygote mice consisting ofAAA andAl<',,A that restrict antigen recognition bycloned Tcells, stim- ulate ML1II responses bycloned Tces, andreact with certain anti- Taantisera. Taantigens werediscovered after purposeful immunization be- tween Irgene-disparate strains ofmiceinanattempt toraise antisera reactive with products ofIrgene loci (1). Almost adec- adehaspassed since theinitial observations concerning Taan- tigens weremade. Despite thenumerous experiments attempt- ingtoidentify immuneresponse geneproducts, ithasnotyet beenpossible toattribute Irphenomena totheserologically demonstrable Taantigens. Manyexperiments havebeenper- formed that demonstrate avery close correlation between im- muneresponse phenomena andTaantigens: forexample, (i) antisera directed atTaantigens block Tcell responses toantigen under thecontrol ofIrgenes (2); (ii) there havebeennore- combinant animals that separate Ialoci andIrloci from thesame subregion ofthe Iregion; (iii) complementing Irgene phenom- enacoincide with complementing Ialoci tocreate newprod- ucts, phenomena, orboth incis- ortrans-complementation be- tween twoseparate subregions oftheIregion (3, 4); and(iv) cells involved inmediation ofIrgenephenomena bearTaan- tigens, asdomanyofthefactors involved inregulation ofim- muneresponses (5). Studies reported below, utilizing themutant B6.C-H-2bml2 (bml2) mouse, will suggest that Taantigens andIrgenephe- nomena, aswell asdeterminants stimulating themixed lym- phocyte reaction (MLR) aredifferent manifestations associated with thesameproduct. Themutant arose spontaneously during studies ofmutation rates (6). Themutation wasofa"gain and loss" type. Complementation studies placed themutation inthe KtoI-Ainterval andtheinability oftheKbmutant strains to complement bml2indicated that themutation waslikely tobe intheI-Absubregion (7). Studies using therecombinant B10. MBR(Kb andI)mapped themutation inbml2within the I-Ab subregion (8). McKenzie etaL(9) could-not detect normal Tab specificities expressed oncells ofthebml2byserologic or biochemical analysis. Despite this there existed two-way MLR andskin graft rejection between thebml2mutant andthepar- entC57BL/6 (B6) strains. Themutant responded normally to thesynthetic polypeptide (TG)-A--L andbovine type IIcolla- genbutwasincapable ofresponding totheloop oftheAchain ofbeef insulin (S. Lin, T.Hansen, andA.Rosenthal, personal communication) orofgenerating cytotoxic Teffector cells tothe H-2-restricted target antigen H-Y(I. McKenzie, personal com- munication). Thedefects exhibited bycells ofbml2arepre- sumably duetoeither aregulatory defect governing synthesis ofI-Apolypeptide chains ortoamutation inadeterminant(s) onapolypeptide that isrequired for normal cell surface expres- sion orbinding.of theachain (Ak), b3 chain (A),andinvariant- chain (I) toformthenormal I-Amolecule (AaAp). Instudies presented below wefound that themutation resulting inloss ofserologically detectable I-Ab products onspleen cells ofbml2 micecanbepartially circumvented byproducing F1hybrids between thebml2mutant andother strains ofmiceexhibiting normal I-Ak geneproducts. Evidence presented inthis paper will show that hybrid I-Aantigens present on(bml2 XB1O.A)F1 cells function asimmuneresponse geneproducts forantigen presentation andarerecognized asalloantigens byalloreactive murine Tcell clones. Datacomparing theresponses ofallo- reactive andsoluble antigen-reactive Tcell clones obtained by using normal (B6A)F1 cells tothose obtained byusing cells of (bml2 XB1O.A)F1 micesuggest that themutation inbml2af- fects notonly I-Ab products butalso trans-complementing (hy- brid) I-Aproducts.