High efficiency of endogenous antigen presentation by MHC class II molecules

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Abstract
MHC class II molecules are Involved in the presentation of both exogenous and endogenous antigens to CD4 T cells. Using the trans-membrane hemagglutinin (HA) from measles virus and the secreted hen egg lysozyme (HEL) as antigen models, we have compared the efficiency of MHC class II presentation by naive antigen presenting cells (APCs) pulsed with exogenous antigen with that of their transfected counterparts synthesizing endogenous antigen. B cells expressing even a very low amount of trans-membrane HA were found to present endogenous HA to I-Ed restricted T cell hybridomas with a high efficiency whereas their naive counterparts required to be pulsed with a comparatively high amount of exogenous HA. Similarly, MHC class II presentation of endogenous secreted HEL was found to be much more efficient when compared with that of exogenous HEL. Biochemical studies did not reveal any enhanced intracellular degradation of endogenous HEL. As expected, HEL was released in the surrounding medium within d molecules immunoprecipltated from HEL secreting cells were found to be slightly enriched In SDS-resistant forms. These data raised the question of how peptides derived from endogenous trans-membrane and secreted antigens can so efficiently reach an MHC class II loading compartment.

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