Myosin-V colocalizes with MHC class II in blood mononuclear cells and is up-regulated by T-lymphocyte activation.

Myosin-V is involved in organelle and vesicle trafficking in Saccharomyces cerevisiae and in other eukaryotic cells from yeast to human. In the present study, we determined by FACS that the major subpopulations of the peripheral blood mononuclear cells express myosin-V with similar fluorescence intensity. Confocal microscopy showed intense labeling for myosin-V at the centrosomal region and a punctate staining throughout the cytoplasm, frequently associated with the central microtubule arrays and the actin-rich cortex. Some degree of overlap with an endolysosomal marker and dynein light-chain 8 k was found at the cell center. Striking colocalization was observed with the major histocompatibility complex (MHC) class II molecules near the cell surface. Treatment with phytohemagglutinin, which induces T-lymphocyte activation, associated with MHC class II expression, increased the levels of myosin-V protein and mRNA for the three members of class V myosins. These data suggest that class V myosins might be involved in relevant functions in the immune response.