The 33 sequences identified
cluster into three major clades with all but one containing mutations in the catalytic triad ruling out the possibility that they can act as proteases by any known mechanism. Two recombinantly expressed scabies mite-inactivated protease paralogues (SMIPPs) were demonstrated as inhibiting all three pathways of the human complement system (83). Both SMIPPs exerted their inhibitory action because of binding of three molecules involved in the three different mechanisms which initiate complement: C1q, mannose binding lectin, and properdin. Both SMIPPs bound to the stalk domains of C1q, possibly displacing or inhibiting C1r/C1s, which are associated with the same domain. learn more The x-ray crystal structures of the two SMIPPs have been determined, (84) but no common structural mode of complement inhibition was apparent. The in vivo effects of these molecules are still unknown, although the decreased Rapamycin levels of C3 and C4 observed in patients with crusted scabies are interesting given the large inflammatory
nature of this condition and could possibly relate to higher levels of SMIPPs expressed by the presence of millions of mites in the skin. Granulocytes are innate effector cells in the host immune defence against many multicellular parasites. Recent emerging data now highlights granulocytes with immunomodulatory roles as well, able to produce cytokines and chemokines that can bias the immune response in a particular direction (85). Eosinophils, mast cells and basophils are reported as responsible for the initiation and ongoing regulation
of Th2 responses. They can be rapidly recruited to sites of infection and draining lymph nodes where they produce IL-4 and/or IL-13 (85). Skin biopsy sections from crusted scabies lesions showed large numbers of infiltrating lymphocytes and eosinophils in the dermis, in conjunction with blood eosinophilia and enhanced production of IgE (4). However, there have been no investigations reported to date on the role and importance of granulocytes in the Th2 biased immune response of crusted scabies. Emerging resistance by scabies mites to currently available chemotherapeutics permethrin and ivermectin highlights the need to identify potential targets for 3-mercaptopyruvate sulfurtransferase chemotherapeutic and/or immunological intervention (10,86–88). Parasite modulation and evasion of host immunity facilitates survival in host tissues and is a critical factor in pathogenicity and transmission. There is much to be gained in understanding the vast and complex array of immunological interactions occurring between parasite and host. Currently, no reliable histological or genetic test is available to determine whether a patient will develop crusted scabies, and hence a definitive diagnosis can often only be determined once a patient has severe disease.