A sheep in wolf's clothes: can neutrophils direct the immune response?

The intercellular transfer of cell membranes and integral membrane proteins has been reported for a wide variety of cells, including cells involved in the immune response, and the passively acquired proteins can alter recipient cell function. Cell membrane transfer can occur by a variety of mechanisms and conditions such as inflammation, cell death, or cell stress increase the release of membrane fragments by donor cells. This review focuses specifically on neutrophils as the recipients of cell membranes and integral membrane proteins. Neutrophils are often the first cells recruited to sites of inflammation where there is ample opportunity to acquire membrane proteins shed by a variety of cells. Our recent investigations have confirmed that bovine neutrophils have an impressive capacity to rapidly acquire membrane proteins from necrotic and apoptotic cells. Furthermore, these acquired proteins can alter neutrophil phenotype and function and we hypothesise that they may enhance their capacity to integrate innate and adaptive immune responses. The implications of these alterations to neutrophil function are discussed within the context of vaccine and novel immune therapy design.

Bovine polymorphonuclear cells passively acquire membrane lipids and integral membrane proteins from apoptotic and necrotic cells.

Immune cells can acquire membrane fragments and integral membrane proteins from dead and dying cells or in the case of immature dendritic cells, from live cells. While investigating the possibility that bovine polymorphonuclear cells (PMNs) might present antigen, coculture assays confirmed that integral membrane proteins were transferred rapidly and efficiently to bovine PMNs from a variety of apoptotic and necrotic cells. Specifically, we observed that PMNs rapidly acquired proteins such as major histocompatibility complex (MHC) class II and CD3 from a variety of syngeneic, allogeneic, and xenogeneic cell types. Such acquisition occurred within 40 min of PMN coculture with isolated peripheral blood mononuclear cells, and this acquisition occurred with equal efficiency at 4 degrees C and 37 degrees C. The transfer of murine MHC class II to bovine PMNs precluded the possibility of endogenous protein expression. We also demonstrated the transfer of fluorescently labeled plasma membrane lipids and biotinylated integral membrane proteins. Collectively, these observations support the hypothesis that membrane protein transfer was mediated by the fusion of membrane fragments or microvesicles with the PMN plasma membrane and not by phagocytosis of cell fragments. These observations indicate that phenotypic studies of PMNs must consider circumstances whereby PMNs may passively acquire membrane lipids and a variety of integral membrane proteins from dead or dying cells.

Pivotal Advance: passively acquired membrane proteins alter the functional capacity of bovine polymorphonuclear cells.

We have previously shown that bovine polymorphonuclear cells (PMNs) have an impressive capacity to passively acquire membrane lipids and proteins from apoptotic cells. The present study used confocal microscopy to analyze the interaction between PMNs and a variety of donor cells, and assays were used to determine if passively acquired membrane proteins altered PMN biology. Confocal microscopy revealed that direct cell-cell contact and microparticles shed by donor cells may be a source of passively acquired membranes and integral membrane proteins, which then integrate into the PMN plasma membrane. Donor cells expressing green fluorescent protein in their cytoplasm were also used to demonstrate the transfer of cytoplasmic proteins from donor cells to PMNs. The functional consequences of passive membrane protein acquisition by PMNs were then investigated using two distinct systems. First, PMNs were incubated with membranes isolated from an adenovirus-permissive cell line, and this passive transfer of cell membranes significantly increased adenovirus infection of PMNs. Second, major histocompatibility complex (MHC) class II molecules were passively transferred from ovine B cells to bovine PMNs, and PMNs with ovine MHC class II on their surface were able to induce a proliferative response and increased cytokine gene expression in alloreactive bovine T cell lines. In conclusion, passively acquired membrane proteins integrated into the plasma membrane of bovine PMNs and altered the functional capacity of these cells.