Analysis of T-cell hybridomas with an unusual MHC class II-dependent ligand specificity

We have characterized two unusual T-cell hybridomas, 1E3 and 3B8, from H-2(k) mice immunized with I-A^b-transfected L cells (H-2(k)), that are stimulated by L cells transfected with I-A^b, I-A(k) or I-E^b, but not by non-transfected L cells. These hybridomas could not be stimulated by spleen cells from H-2(i3), H-2(k), H-2^b or H-2(d) mice. Monoclonal anti-I-A antibodies did not block their responses, suggesting that mouse major histocompatibility complex (MHC) class II molecules may be peptide donors rather than restriction elements for them. The stimulation of these hybridomas by fibroblast targets was not blocked by an anti-H-2k(k),D(k)-specific monoclonal antibody. Lipopolysaccharide (LPS)-activated splenic and peritoneal exudate cells from H-2(k), H-2(d), H-2(i3), H-2^b as well as β₂-microglobulin-deficient, TAP-1-deficient and I-Aα-deficient H-2^b mice stimulated these hybridomas. LPS could also activate a macrophage cell line, but not a B-cell line, to become stimulatory for 1E3. A rat antiserum against untransfected L cells specifically and significantly blocked the response of 1E3. Thus, 1E3 may recognize a conserved murine MHC class II peptide loaded in a TAP-1-independent fashion on a non-classical, monomorphic, β₂-microglobulin-independent restriction element.