Biochemical characterization of B lymphoma cell antigen processing and presentation to antigen-reactive T cells.

Generating an immune response to T-dependent antigens requires the cooperative interaction of APC, T helper cells, and B cells. T helper cells recognize antigen, not as soluble, native antigen, but as processed antigen in association with Ia molecules on the surface of an APC. Investigators from our laboratory demonstrated previously that some B lymphoma cell lines will present antigen to antigen-reactive T cells in an MHC-restricted fashion. These tumor lines are used in this study as a model system to examine the biochemical basis of antigen processing. Five different H-2d tumor cell lines, which differ in their ability to express Ia molecules and function as APC, are all shown to biochemically degrade sperm whale Mb in a similar manner. Intact Mb and four Mb fragments (4.3 to 12.5 Kd) were reproducibly recovered from each of these Mb-pulsed cell lines. This processed Mb is shown to be nonrandomly distributed in external and internal compartments, with the two smallest Mb fragments being enriched in the intracellular compartment. The Mb and Mb fragments can be completely removed from the surface of the APC by enzymatic proteolysis without quantitatively changing the ability of these cells to present Mb to Mb-reactive T cell lines. These results suggest that the processed Mb that is located intracellularly is the primary immunoreactive antigen for antigen presentation to T helper cells.