Different phenotypic variants of the mouse B cell tumor A20/2J are selected by antigen- and mitogen-triggered cytotoxicity of L3T4-positive, I-A-restricted T cell clones

The L3T4+, Lyt-2-, cloned BALB/c T cell lines 5.9.24 and 5.8.6 are cytotoxic for the BALB/c B cell tumor line A20/2J. The T cell cytotoxicity against A20/2J cells could be triggered either by the specific antigen ovalbumin (OVA), which is recognized by the T cell clones in association with I-Ad determinants, or by the T cell mitogens Con A and rabbit anti-mouse brain (RaMBr) antiserum. Repeated exposure of A20/2J cells to 5.9.24 and 5.8.6 T cell cytotoxicity selected variant cell lines that had developed resistance to cytotoxicity. The variant lines could be classified into four different variant phenotypes of which three were stably maintained in vitro. The type of variant obtained appeared to be related to the nature of the ligand used to trigger T cell cytotoxicity during selection. Cytotoxicity triggered by the antigen OVA generated type 1 variants that expressed abnormally low levels of I-Ad determinants at the cell surface. Type 1 variants were resistant to OVA-triggered 5.9.24 T cell cytotoxicity, but were fully susceptible to cytotoxicity triggered by Con A or RaMBr antiserum. RaMBr-triggered cytotoxicity generated two unique types of variant cell lines: type 3 variants that were deficient in cell surface Fc receptors and resistant to 5.9.24 cytotoxicity only when triggered by RaMBr antiserum, and type 4 variants that were resistant to cytotoxicity triggered by all three ligands. One type 4 variant, the IC-1 cell line, appeared to be resistant to soluble cytotoxic factors released by 5.9.24 T cells after activation by antigen. All of these variant lines retained sensitivity to cytotoxicity by classic Lyt-2+ cytotoxic T lymphocytes (CTL), a finding that indicates that L3T4a+ T cells and Lyt-2+ CTL use different molecules to attack their target cells. The variant phenotypes were inherited by clones derived from the original cell lines. Because the variants were generated without mutagenesis, they are thought to have been derived by the immunoselection of pre-existing variant cells that arose spontaneously in the parental A20/2J cell line. It is postulated that inheritable variation of A20/2J cells may represent changes that normally occur during B cell differentiation in response to T cell signals. The variant A20/2J cell lines described here provide material for the investigation of B cell surface structures that may regulate T-B cell interactions.