The cellular and humoral immune responses of adult American cockroach, Periplaneta americana to the Egyptian entomopathogenic nematode Steinernema sp. 2D were investigated. Total and differential haemocyte counts from Steinernema-injected, water-injected, and uninjected P. americana were determined using a light microscope. Four types of haemocytes were identified in control P. americana: prohaemocytes, plasmatocytes, granulocytes and coagulocytes. Following challenge with infective juveniles of Steinernema sp. 2D, the symbiotic Xenorhabdus bacterium was observed in haemolymph smears. Few bacteria were phagositozed by plasmatocytes. Haemocyte clumps entrapping bacteria were formed, but typical nodules were not observed. Prohaemocytes of infected cockroaches showed membrane blebbing, vacuolated cytoplasm and pycnotic nuclei. Granular haemocytes lysed during infection and released their granular inclusions. Nuclear chromatin condensation and haemocytes with two nuclei were observed. Coagulocytes were vacuolated and their nuclei exhibited nuclear lesion. Steinernema injection resulted in dramatic decrease in the proportion of plasmatocytes, coagulocytes and granulocytes 12 h post injection and significant increase in the proportion of prohaemocytes. By 24 hours post nematode challenge, the proportion of plasmatocytes and coagulocytes retained to increase, meanwhile, the percentage of prohaemocytes was still significantly higher than those of uninjected and water injected cockroaches. On the other hand, the proportion of granulocytes remained significantly lower compared with the uninjected and water injected cockroaches. Nematode injection decreased the total haemocyte count by 12 h post injection followed by an increase to control level at 24 h post injection. Phenoloxidase (PO) activity in the plasma of nematode-injected cockroaches significantly increased at 12 h post injection followed by a decrease at 24 h post injection. Both activities were significantly higher than those detected in uninjected and water-injected P. americana plasma. The results suggested that Steinernema bacterial complex possesses an efficient immune suppressive mechanisms resulting in specticemia and rapid death of the American cockroach.
Periplaneta, Steinernema, Immunity