Pamela A. Reid

Processed antigen binds to newly synthesized mhc class II molecules in antigen-specific B lymphocytes

We describe the direct detection of radiolabeled antigen fragments bound to class II MHC molecules following immunoglobulin-mediated endocytosis and processing of native antigen in B lymphoblastoid cells. Tris-Tricine SDS gels revealed six distinct iodinated processing products that could be detected on class II MHC 1 hr after antigen endocytosis and persisted for at least 20 hr. These physiological processed antigen-class II complexes were remarkably stable, as judged by the fact that class II alpha beta dimers, which remain associated in SDS, became labeled with the same set of processed peptides. Using a lectin-binding assay, we show that these physiological processing products bind to the newly maturing population of MHC molecules rather than binding to the preexisting cell surface population; in contrast, an exogenous peptide binds predominantly to the latter population. A direct T cell-independent assay for processed peptide-MHC complex formation should facilitate additional studies on the exogenous antigen processing pathway.

The antigen receptors on mature and immature T lymphocytes are coupled to phosphatidylcholine‐specific phospholipase D activation

Phosphatidylcholine–phospholipase D has been proposed to play a key role in the transduction of the proliferative responses of a wide range of mitogens and growth factors. We now report that the antigen receptors on T lymphocytes derived from human tonsillar or murine splenic preparations are coupled to phosphatidylcholine (PtdCho)–phospholipase D (PLD) activation following stimulation of these T cells with anti‐CD3 antibodies. However, since we also demonstrate that the antigen receptors on murine thymocytes are coupled to PtdCho–PLD activation, we propose that it is unlikely that this PLD pathway plays a central role in the transduction of T‐cell proliferative responses, but rather, may be involved in either driving cells into cycle or maintaining cell cycle progression, processes required both for proliferation and activation‐induced cell death. Whilst the molecular mechanisms underlying T‐cell receptor (TCR)‐coupling to PtdCho–PLD activation in these cells have not been fully defined, kinetics studies and experiments using pharmacological inhibitors of protein tyrosine phosphatases (PTPases) and reconstituting CD3‐coupled PtdCho–PLD activity in streptolysin‐O permeabilized cells, suggest that the TCR/CD3 complex, under optimal conditions of activation, may be predominantly coupled to PtdCho–PLD activation downstream of tyrosine phosphorylation of phospholipase C‐γ (PLC‐γ), phosphatidylinositol (PtdIns)P2hydrolysis, calcium mobilization and protein kinase C (PKC) activation.

Antigen Processing in B Lymphocytes

There is considerable current interest in how the events occurring during antigen processing and presentation via class II MHC glycoproteins are organised with respect to the endocytic pathway. This follows the discovery that most antigens require an intracellular processing step in order to produce a suitable ligand for binding to MHC Class II glycoproteins and, as a complex, to the T cell receptor. The available evidence strongly suggests that following endocytosis, processing and unfolding take place in intracellular compartments and that suitable MHC/peptide complexes are then returned to the cell surface. Using T cells to assay for the appearance of processed determinants it has been established that these antigen processing events take approximately one hour, are sensitive to chloroquine and fixation of the antigen presenting cells (APC) and to inhibitors of endosomal/lysosomal proteases (Ziegler and Unanue 1982; Lanzavecchia 1985; Buus and Werdelin 1986).