Early responses of primary human and bovine monocytes, monocytic THP-1 cells and THP-1 cell-derived macrophages to vital Toxoplasma gondii tachyzoites

Microscopic analysis illustrated the activation of all cell types tested within 4 h of exposure to <i>T. gondii</i> tachyzoites. Numerous tachyzoites were found intracellularly in THP-1 cell-derived M1-like macrophages. Furthermore, the co-localization of extracellular DNA (extDNA) and histones in extracellular web-like fibers proved classical characteristics of extruded <i>T. gondii</i>-induced METs, although this was a rare event. In primary human monocytes, an increased release of IL-1β and IL-6 was observed following exposure to <i>T. gondii</i> tachyzoites. When co-stimulated with lipopolysaccharide (LPS), primary human monocytes showed an enhanced release of IL-1β and IL-6 in response to <i>T. gondii</i>. In contrast to monocytic THP-1 cells, THP-1 cell-derived M1-like macrophages released IL-1β in response to <i>T. gondii</i> tachyzoite exposure. When additionally stimulated by LPS, all THP-1 cell-derived macrophages showed an enhanced release of IL-1β, and monocytic THP-1 cells an increased release of IL-6 in response to <i>T. gondii</i> tachyzoites. This study provides insights into the early innate immune response of human and bovine mononuclear phagocytes to <i>T. gondii</i>. While cytokine secretion was prominent, MET formation was rare in the early response (i.e. < 4 h of exposure) to <i>T. gondii</i> tachyzoites.