We previously noted that Mlsa,cC58/J responder cells proliferated unexpectedly to H-2k-compatible Mlsaor Mlscprototypic stimulator cells in a primary mixed lymphocyte reaction. The present investigation was performed to evaluate whether the response of C58/J T cells to these H-2- and Mls-compatible stimulator cells could functionally identify a newly-defined member of the Mls superantigen family through its allostimulatory ability. We observed that C58/J responder cells also proliferated when cultured with H-2-compatible prototypic Mlsnull, Mlsb(nonstimulatory), or Mlsa,csplenic stimulator cells. The widely distributed nature of the non-MHC ligand recognized by C58/J T cells is indicated by the finding that 11 of 12 H-2kinbred mouse strains clearly expressed this specificity. A gradient of stimulatory capacity from low to high across this newly-defined non-MHC difference was detected with splenocytes from these different inbred mouse strains. The Mlsa,cgenetic composition of C58/J was confirmed by the observation that crossing C58/J with parental B10.BR (responsive to both Mlsaand Mlscdeterminants) generated F1progeny that were unresponsive to H-2k-compatible Mlsa, Mlsc, or Mlsa,cstimulator cells. Like prototypic Mlsaand Mlsc, the non-MHC specificity recognized by C58/J responder cells, termed Mlsf, was particularly sensitive to radiation (versus mitomycin C) treatment of the stimulator cells, was greatly augmented after anti-IgD activation of splenic stimulator cells, was blocked with anti-MHC class II antibody, and was effectively presented by phenotypically normal female but not B cell-defective xid+male CBA/N F1stimulator cells.
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