Synthesis and cell surface display of class II determinants by long‐term propagated rat T line cells

Abstract We have investigated the capacity of the encephalitogenic BS rat T cell line bs 83 and its variant clone bs83.III.C6 to synthesize and express RT1.B‐specific class II molecule subsets defined by monoclonal antibodies (mAb) MRC‐OX6 and MRC‐OX3. Earlier studies had indicated that mAb MRC‐OX6 recognizes three distinct molecular species: an immature oligomeric polypeptide chain complex comprised of the polymorphic subunits α,β and the invariant proteins of the γ group; a biosynthetic intermediate composed of post‐translationally modified α,β and γ chain (denoted p35) and a fully glycosylated α,β two‐chain complex derived from the plasma membrane. MRC‐OX3 was shown to recognize a serologically distinct α,β two‐chain complex that coexists with the MRC‐OX6‐specific heterodimer at the cell surface. Here we show that premutant bs83 cells were unable to synthesize class II molecules of either set. In contrast endogeneous synthesis by mutant cells of MRC‐OX6‐specific molecules was demonstrated. Unlike control spleen cells variant cells failed to synthesize the mature MRC‐OX3‐reactive class II subset. Instead a three‐polypeptide chain complex comprised of the terminally glycosylated subunits α,β and invariant chain p35 was present at the cell surface. This complex appears to represent the preserved biosynthetic intermediate that failed to release invariant chain p35 upon its transit into the plasma membrane. These latter observations support our notion of γ chain‐induced epitope diversification during post‐translational maturation of RT1.B‐specific class II molecules.