Immunologic Capping

Abstract: The signaling pathways that lead to the localization of cellular protein to the area of interaction between T cell and antigen-presenting cell and the mechanism by which these molecules are further sorted to the peripheral supramolecular activation cluster or central supramolecular activation cluster regions of the immunologic synapse are poorly understood. In this study, we investigated the functional involvement of CD28 costimulation in the T cell receptor (TCR)-mediated immunologic synapse formation with respect to protein kinase C (PKC)θ localization. We showed that CD3 crosslinking alone was sufficient to induce PKCθ capping in naive CD4+ T cells. Studies with pharmacologic inhibitors and knockout mice showed that the TCR-derived signaling that drives PKCθ membrane translocation requires the Src family kinase, Lck, but not Fyn. In addition, a time course study of the persistence of T cell molecules to the immunologic synapse indicated that PKCθ, unlike TCR, persisted in the synapse for at least 4 h, a time that is sufficient for commitment of a T cell to cell division. Finally, by using TCR-transgenic T cells from either wild-type or CD28-deficient mice, we showed that CD28 expression was required for the formation of the mature immunologic synapse, because antigen stimulation of CD28− T cells led to a diffuse pattern of localization of PKCθ and lymphocyte function-associated antigen-1 in the immunologic synapse, in contrast to the central supramolecular activation cluster localization of PKCθ in CD28+ T cells.

Abstract: A detailed kinetic analysis of the distribution of cytoplasmic myosin during the capping of various lymphocytic surface molecules revealed two distinct capping mechanisms. (a) Some cell surface molecules, including immunoglobulin, Fc receptor, and thymus leukemia antigen, all cap spontaneously in a small fraction of lymphocytes during locomotion. Cytoplasmic myosin becomes concentrated in the cytoplasm underlying these spontaneous caps. Exposure to specific antibodies causes all three of these surface molecules to cap rapidly with a concomitant redistribution of cytoplasmic myosin to the area of the cap. These antibodies also stimulate cell locomotion. (b) Other lymphocyte surface molecules, including H2 and Thy.1, do not cap spontaneously. Moreover, exposure to antibodies to these molecules causes them to cap slowly without a redistribution of cytoplasmic myosin or stimulation of cell locomotion. Exposure to concanavalin A gives a response intermediate between these two extremes. We believe that the first type of capping is active and may involve a direct link between the surface molecules and the cytoplasmic contractile apparatus. The second type of capping appears to result simply from aggregation of cross-linked molecules in the plane of the membrane.

Abstract: The capping of surface Ig on B cells occurs with a striking redistribution of cytoplasmic myosin Our results suggest a close association between surface Ig and myosin which could be the basis for Ig redistribution and stimulated motility