Abstract
The complement anaphylatoxin C5a is a critical mediator of innate immunity and inflammation. Two G-protein coupled receptors (GPCR) have been described to bind C5a: The C5a receptor (C5aR/C5aR1) and the C5a receptor like-2 (C5L2/C5aR2). In contrast to C5aR1, the role of C5aR2 in C5a-mediated responses remains unclear.
In our study we wanted to quantify the expression and activation of both receptors in primary human blood cells of myeloid origin, with focus on granulocytes versus monocytes.
For that purpose we treated leukocytes obtained from healthy donors with recombinant human C5a (and C5a-desArg). Expression of C5aR1 and C5aR2 was quantified before and after treatment by FACS analyses. Before treatment, expression of both receptors could be confirmed on both cell types. Upon activation, GPCRs are internalized and therefore cell surface expression is expected to be decreased. Indeed, decreased expression of C5aR1 was demonstrated after treatment with C5a on both granulocytes and monocytes. Unexpectedly, C5aR2 was internalized on granulocytes upon treatment with C5a, but not on monocytes. This lack of internalization of C5aR2 on monocytes was also observed in the monocytic cell line, U937. Moreover, we demonstrated, that radioactive labelled C5a binds to C5aR2 expressed on stable transfected CHO cells, but does not bind to C5aR2 expressed endogenously on U937 cells. In summary we showed that expression of C5aR2 on monocytes is not sufficient for activation (binding and internalization) of this receptor. We conclude that the accessibility of C5aR2 is regulated in cell type specific manner.
- Copyright ©the authors 2016