Presentation of IgG2a antigens to class II-restricted T cells by stably transfected B lymphoma cells

Here we describe a panel of BALB/c T cells specific for IgG2a of the b allotype in association with I-Ad. We used DNA-mediated gene transfer techniques to localize antigenic determinants recognized by responding T cells. Initially a truncated IgG2aa gene comprising a variable domain and the CH3 domain (not including the membrane exons) from the BALB/c IgG2aa heavy chain was introduced into myeloma cells. The V-CH3 protein was expressed at high levels under control of the Ig heavy chain enhancer. Secretion of the V-CH3 protein did not require assembly of H-H dimers or an association with light chains. To generate stably transfected B cell lines that would stimulate our class II-restricted T cells, we replaced most of the BALB/c IgG2aa CH3 exon with CH3 coding sequences from a C57BL/6 IgG2ab cDNA clone and introduced these constructs into Ia+ B lymphoma cells. The IgG2ab CH3-transfected B cells were recognized by BALB/c Igh-1b-specific T cell hybrids in the absence of exogenous antigen. Experiments using glutaraldehyde-fixed cells as stimulators indicate that presentation of the secreted form of V-IgG2ab CH3 requires processing. We found that a significant fraction of the endogenously synthesized V-IgG2ab CH3 protein was, however, present as already processed antigen.