Pythium insidiosum-antigen enhances neutrophil-mediated killing of zoospores

Pythium insidiosum-antigen (PIA) immunotherapy has been used to treat human pythiosis. This study compared PIA-stimulated and unstimulated neutrophils on zoospore viability of P. insidiosum strains. We cultured and collected zoospores of 6 P. insidiosum strains, CBS 777.84, ATCC 58643, ATCC 90586, PEC1, PC10, and CBS 101039. PIA concentrations of 0.01, 0.1, 1, and 10 µg/ml were prepared and were used to stimulate neutrophils isolated from healthy volunteers. The MTT (3-(4, 5-dimethylthiazolyl-2)-2, 5-diphenyltetrazolium bromide) assay was used to calculate the zoospore viability of P. insidiosum strains. Neutrophils stimulated with 0.01 or 0.1 µg/ml PIA, or both, showed a significant reduction in the viability of zoospores of CBS 777.84, ATCC 58643, CBS 101039, PC10, and PEC1 strains. Furthermore, 1 µg/ml PIA-induced neutrophils elicited a significant decrease in the viability of zoospores of ATCC 58643, CBS 101039, and PC10 strains. However, a higher dose of PIA (10 µg/ml) did not demonstrate superiority in reducing the zoospore viability of all six strains. Our findings suggest that PIA immunotherapy improves the zoospore-killing activity of neutrophils, and neutrophils might be involved in the forefront mechanism responsible for the beneficial effects of PIA immunotherapy.