CELL-SURFACE EXPRESSION OF HUMAN HISTOCOMPATIBILITY LEUKOCYTE ANTIGEN(HLA) CLASS-II-ASSOCIATED INVARIANT CHAIN (CD74) DOES NOT ALWAYS CORRELATE WITH CELL-SURFACE EXPRESSION OF HLA CLASS-II MOLECULES

The human histocompatibility leucocyte antigen (HLA) class II-associat ed invariant chain comprises at least four polypeptides. One of these is expressed as a membrane-bound subunit and has been designated as CD 74. We investigated the expression of CD74 on B- and T-cell types util izing monoclonal antibodies (mAb) specific for the c-terminus of CD74. All B cells tested expressed CD74 regardless of their HLA phenotypes. High and low CD74 expressors were provisionally assigned based on qua ntitative variations found in some haplotypes. A mutant cell with sele ctive deletions of class II loci DR and DQ but with retention of DP an d additional class II heterodimers synthesize normal amounts of CD74. In contrast, however, a mutant with a total class II deletion had dimi nished expression of CD74, suggesting that at least one of the class I I isotypes is essential for full constitutive expression of CD74. Majo r histocompatibility complex (MHC) class II negative T-cell lines and CD3+ T cell isolated from peripheral blood did not express CD74. Howev er, allostimulated T cells which express de novo induced class II dete rminants also did not express CD74. Taken together, these results show that CD74 is expressed on B cells and unexpectedly is not synthesized on de novo-induced class II positive T-cell clones.