Cell contact-dependent PMN HLA-DR and CD69 membrane expression induced by autologous mono-lymphocytes and cell lines

Polymorphonuclear granulocytes (PMN) are commonly considered short-lived cells playing an efficient role in primary host defense via phagocytosis and release of cytotoxic compounds and inflammatory cytokines. Purified PMN do not express HLA-DR and CD69 molecules on cell surface, but they can be induced to do so by co-culture with peripheral blood derived mono-lymphocytes. De novo cell-surface expression of HLA-DR was also induced in PMN by co-culture with cell lines of lymphoid phenotype, but not with cell lines of myeloid phenotype. CD69 expression was not induced by co-culture with any of the cell lines used in the present study. In addition, we have observed induction of HLA-DR surface expression on PMN by culture in presence of culture supernatant of one of the cell lines of lymphoid origin, RPMI-8866. Quantitative analysis of HLA-DR and CD69 expression in stimulated PMN allowed us to divide PMN donors in two main groups, one with low expression and the other with high expression of the two molecules. HLA-DR surface expression was not altered by treatment with CHX and BFA, and RT-PCR analysis of total RNA from resting and stimulated PMN with RPMI-8866 supernatant did not detect the presence of any specific HLA-DR and CIITA transcript. Flow-cytometry and fluorescence microscopy analysis of resting PMN revealed the presence of HLA-DR molecules localized in intracellular vesicular-tubular structures. These data show that a reservoir of HLA-DR molecules is stored in the cytoplasm of human resting PMN and can be released to reach cell surface by a mobilization mechanism induced by cell surface interactions with selected cell types and sometimes with molecules released in culture supernatants.