Differential expression of the invariant chain in mouse tumor cells: relationship to B lymphoid development.

We have examined 25 cultured lines of mouse tumor cells for synthesis of the Ii, an Ia-associated polypeptide, by using an anti-Ii monoclonal antibody. Six of the T lymphomas tested did not produce detectable levels of Ii or of surface Ia antigens. Three B lymphomas and two plasmacytomas that express surface Ia antigens were found to synthesize the Ii. In addition, Ii was immunoprecipitated from two of five Ia- pre-B lymphomas, two of four Ia- plasmacytomas, two Ia- myeloid tumors, and two fibroblast cell lines including LM(TK-). Because Ia antigens have so far been found only on cells that also synthesize Ii, we suggest that the Ii is a marker of those cells that in certain states of development or activation express Ia antigens.