Differential Effects of Serum Heat Treatment on Chemotaxis and Phagocytosis by Human Neutrophils

Abstract

The ability of certain white blood cells to actively change their shape is not only a cornerstone of a host's innate immune defense, but also provides a highly instructive, cross-disciplinary window into eukaryotic cell motility. Recent findings have indicated that human neutrophils rely at least partially on similar biophysical mechanisms during chemotaxis and phagocytosis. To further elucidate similarities and differences between these two prominent examples of immune-cell motility, we here examine the effects of heat treatment of serum components on neutrophil interactions with antibody-coated beads and zymosan particles. Because heat treatment can inhibit some or all viral activity in the serum, it is a common method to protect laboratory personnel, and also has many other applications. Here, we incubate neutrophils with the two target types in buffers containing serum that had been treated at a range of different temperatures. To discriminate between phagocytic and chemotactic behavior, we quantify the cell response by combining flow cytometry bulk assays with single-cell/single-target experiments based on dual-micropipette manipulation. We find that heat treatment of serum at 52°C or higher significantly depressed phagocytosis of opsonized zymosan in our bulk assays for all donors tested. On the other hand, single-cell experiments demonstrated complete inhibition of chemotactic activity already at 48°C, even though the neutrophils still engulfed target particles that were brought into physical contact with the cells using micropipettes. This implies that, although chemotactic stimulation of neutrophils is not required for phagocytosis, it can significantly enhance the phagocytic response. Our results also demonstrate that by fine-tuning heat treatment of serum, one can selectively study chemotaxis or phagocytosis under otherwise identical conditions.