Spectrin immunofluorescence distinguishes a population of naturally capped lymphocytes in-situ

The physiological significance of the lymphocyte capping phenomenon is unkown. This is due primarily to a lack of observations of capping within lymphoid organs. Examination of frozen sections of mammalian lymphoid tissues after immunofluorescent staining with anti-chicken erythrocyte α-spectrin antisera, reveals a distinct population of lymphocytes in which spectrin antigen is arranged in the form of a discrete cap. In the thymus of one week-old mice, nearly 100% of the medullary lymphocytes have the capped configuration of spectrin whereas in the cortical region, the lymphocytes are almost entirely uncapped, i.e., the plasma membranes are associated with uniformly distributed spectrin. In the spleen, capped lymphocytes are found in colonies which are restricted to the white pulp. In Peyer's patches, the capped lymphocytes are diffusely dispersed throughout the tissue, and in lymph nodes they are found predominantly in the paracortical region. Capped lymphocytes are not visible in the thymus until just before birth and do not appear in the spleen until 3 days after birth. Since lymphocyte spectrin has been found by others to be associated with several crosslinked surface macromolecules in co-capping experiments using isolated lymphocytes, it will be of interest to determine which, if any, of these surface macromolecules may be naturally associated with spectrin in a capped configuration.