Innate recognition and phagocytosis of zymosan by human neutrophils require cell priming in a p38-dependent manner

Neutrophils are the first line of defense against fungal infection and other microbial pathogens. Through alternative pathway of complement activation in innate immunity, complement fragments (opsonin) generated promote phagocytosis of the yeast forms of fungal pathogens such as Candida albicans. However, when complement levels are limited, through consumption in disease and inflammation or where microbial pathogens employ mechanisms to degrade or inhibit complement components, phagocytosis may still proceed via innate receptors, in the absence of effective opsonisation. Mechanisms of innate phagocytosis of fungi via neutrophils remain ill-defined and controversial. We have addressed this issue and demonstrated that neutrophils show very little phagocytosis of zymosan. Interestingly, activation or priming of neutrophils with phorbol-12-myristate-13-acetate, N-formyl-methionyl-leucyl-phenylalanine, arachidonic acid, tumor necrosis factor, and granulocyte macrophage-colony stimulating factor led to significant phagocytosis of the fungi. The result also demonstrated that priming led to a significant increase in the expression of the neutrophil Dectin-1, a receptor for β-glucan on zymosan. The increased phagocytosis induced by priming could be blocked with an anti-Dectin-1 antibody as well as the soluble β-glucan, laminarin, suggesting the importance of Dectin-1 in the phagocytosis of zymosan. The priming mechanism for phagocytosis and upregulation of Dectin-1 was examined using pharmacological inhibitors of protein kinase C (PKC) and p38 MAP kinase activation. The results showed that priming occurs in a PKC- p38-dependent as well as a PKC-independent p38-dependent manner. The findings demonstrate a highly regulated innate immune mechanism that potentially rapidly tackles microbial pathogens invading the bloodstream and tissues.