Influence of dopamine on the cellular immune response of Rhipicephalus microplus infected with Metarhizium anisopliae

Entomopathogenic fungi, bovine tick, immune response

Currently, the control of Rhipicephalus microplus is carried out through the use of chemical acaricides, but the misuse of these products contributes to the development of ticks’ populations more resistant, contamination of animal products and the environment by their residues. In this situation, biological control by entomopathogenic fungi can be used in integrated management to control ticks. Metarhizium spp. is one of the fungi used with recognized performance in the biological control of arthropods. Hemocytes, cells present in hemolymph, are involved in the immune response and their phagocytic action is considered important in the immune response of ticks against pathogens. However, a greater understanding is needed of the role of the R. microplus immune system and its defense mechanisms against a fungal infection is needed to advance in the development of more effective control control against the parasite. Dopamine (DA) is a biogenic monoamine that interconnects the nervous and immune systems, reported to modulate the phagocytosis of insect hemocytes. The present study evaluated the effect of DA in the survival of ticks treated with M. anisopliae and its role in the cellular immune response of R. microplus ticks infected with entomopathogenic fungi. Fully engorged R. microplus females were inoculated according to the groups: CTR (untreated group), PBS (ticks inoculated with phosphate buffer solution), DA (ticks inoculated with 3μL of 1025 ng μL-1 DA), Ma [ticks inoculated with M. anisopliae (3μL; 106 blastospores mL-1)], and Ma + DA [ticks inoculated with DA (3μL; 1.025 ng μL-1) and after 20 minutes, M. anisopliae (3μL; 106 blastospores mL-1)]. And only CTR, PBS and DA for the phagocytosis assay. By analyzing the effects on susceptibility to the fungus, exogenous DA increased female survival after being inoculated with dopamine followed by fungus 72h after treatment, increased the number of circulating hemocytes in the DA group followed or not by fungal infection 24h after inoculation and it was possible to detect it in hemocytes of treated and untreated ticks with fungus. However, we did not find data that demonstrate the influence of DA on the phagocytic index of hemocytes of ticks challenged with M. anisopliae and the activity of phenoloxidase in the hemolymph of ticks was similar between ticks treated exclusively with the fungus and ticks treated with DA and the fungus. The immune response analysis of the of R. microplus challenged with entomopathogenic fungi contributes to the advancement and development of new effective biological control. Our study multiplied for the first time the modulation of DA in the cellular immune response of R. microplus infected with M. anisopliae.