Innate Immunity in Ticks: A review

Ticks transmit mere different kinds ef pathogenic organisms than any other hematophagous arthropod. This paper attcmpts to review the literature available on the cell-mediated and humoTa] responses ofticks. The eell-mediated defense responses by invertebrates are impler than vertebrates, but are elTectivc as a second line ofdefense against invasion by microorganisms. Hemocytes play a major role in this immune response through phagocytosis, nodule fonnation and encapsulation of invading fbreign materials. These mechanisms appear to fi]nction i both soft and hard ticks. Humoral immune responses consist of three main typcs; humoral encapsulation, hemagglutination and production of antimicrobial proteins. Hurnoral encapsulation is elicited by the prophcnoloxidasc activating system and has only been s]iown to function in the soft tick Ornithodoros moubata. IIowever, rcccnt studies shuw tbehard ticks also have prophenoloxidase activating cofactors indieating humoral encapsulation aiso functions in these ticks. Tick lcctins function in hemagglutination of various microorganisms in both soft and hard ticks and also appear to function as recogriition molecules ofnonselfandpossibly in the transmission ofpathogenic microorganisms. Ticks also have numerous antimicrobial proteins that can attack the invading microorganisms. fysozymes have been shown to have bactericidal etfects in several tick species and work in concert with other antimicrobial peptides to ki11 invaders, Cystatins, classic cysteinc proteasc inhibitors, have recently bccn shown to f-unction in innate immune responses of ticks, Hemoglobin fragments have antimicrobial activity in the midgut of ticks, Defensins show expression up-regulated by blood f'eeding and are efft]etive antimicrebial agents with some diflk rences between soft and hard ticks. Other novel antimicrobial peptides have been isolated from various species ofsoft and hard ticks, but thcir functions remain to be elucidated. Understanding these innate immune mechanisms in ticks is essentiaL to develop strategies for reducing the transmission ofdiseases to man and animals, Key vvords: eell-mediated immunity, humeTal immunity, ticks, phagocytosis, hemocytes, encapsulation, lectins, lysozyme, defensin, hernoglobin fragments