High CD99 expression in memory T and B cells in reactive lymph nodes

We investigated the expression of CD99 in 35 hyperplastic perigastric lymph nodes, which were resected for gastric carcinoma or chronic peptic ulcer. Essentially, all lymphocytes in lymph nodes expressed CD99, but there were two populations with respect to the intensity of CD99 expression - CD99(high) and CD99(low) cells. We showed CD99(high) cells were distributed in paracortical and medullary cords by immunohistochemical study while germinal center cells were CD99(low). Using three-color flow cytometric analysis with CD3, CD4, CD8, CD19, CD23, CD45RA, CD45RO, CD69, CD138, IgM, IgD, and IgG, most of CD99(high) cells were shown to be activated/memory T cells. CD4(+)CD45RO(+) T cells were the subset revealing the highest intensity of CD99 expression while CD4(+)CD45RA(+) T cells were CD99(low). Among B cells, IgG(+) B cells revealed a higher level of CD99 molecules than IgM(+) B cells. These results suggest that CD99 is one of activation-related molecules which are upregulated in recently activated lymphocytes.