Parasitic infections in man and domestic animals exhibit two striking characteristics (a) their prevalence is high, but infections are unequally distributed among individuals within populations and (b) immunity is often slow to develop and appears, at best, only partially effective. Recent immunological and epidemiological studies suggest that effective immunity can develop, but that high prevalence within populations reflects the operation, not only of socio-economic and climatic factors, or husbandry practices, but also of powerful environmentally induced constraints upon the development of resistance. Immunogenetic studies suggest the operation of additional constraints which reflect individual genetic characteristics, and which influence the ability to develop and express effective immunity. A full understanding of all constraints is necessary before levels of population and individual resistance to infection can be increased; the need for such understanding has become more pressing with the prospect that anti-parasite vaccines may become available. Two aspects of environmentally induced constraints are considered, those arising from nutritional inadequacies and those resulting from exposure to infection in early life. Both are discussed primarily in terms of helminth parasites. Genetically determined constraints are discussed with reference to MHC-restricted recognition of malarial peptide vaccines and in terms of Class II molecule-directed control of T-cell function in Leishmania infections. Genetic influences are also considered from the standpoint of inflammatory cell function, in immunity against intestinal nematodes and in vaccine-induced immunity against Schistosoma. Finally, parasite-induced constraints, particularly those which down-regulate protective responses are discussed briefly.

In the present study, the expression of the CD4 molecule on murine egg plasma membrane was confirmed by the indirect immunofluorescence (IIF) method. The full-length CD4 cDNA from murine eggs was synthesised by the reverse transcriptase-polymerase chain reaction (RT-PCR) method and its authenticity verified by Southern blot hybridisation using an end-labelled internal oligonucleotide. The results of DNA sequencing showed that the nucleotide sequence of the cDNA of CD4 from murine egg mRNA was identical to that of immune T cells. To demonstrate the direct interaction of CD4 from murine egg with murine sperm cells bearing MHC (major histocompatibility complex) class II molecule, we employed a baculovirus expression system to generate CD4 on the surface of Spodoptera frugiperda (Sf9) cells. Expression of CD4 on Sf9 cells infected with autographa californica nuclear polyhedrosis virus (AcNPV)-CD4 was demonstrated by IIF and immunoblotting. The CD4-expressing Sf9 cells adhered to MHC class II-bearing sperm cells since the adhension was specifically blocked by anti-CD4 monoclonal antibody (mAb) or anti-monomorphic region of MHC class II mAb. Taking our previous and present experimental results together, they strongly suggest that intercellular membrane adhesion between two gametes at the fusion step in fertilisation is mediated by the MHC class II molecule located on the posterior region of the sperm head and the CD4 molecule on egg plasma membrane.

The protozoa Leishmania spp. are obligate intracellular parasites that inhabit the macrophages of their host. Since macrophages are specialized for the identification and destruction of invading pathogens, both directly and by triggering an innate immune response, Leishmania have evolved a number of mechanisms for suppressing some critical macrophage activities. In this review, we discuss how various species of Leishmania distort the host macrophage's own signalling pathways to repress the expression of various cytokines and microbicidal molecules (nitric oxide and reactive oxygen species), and antigen presentation. In particular, we describe how MAP Kinase and JAK/STAT cascades are repressed, and intracellular Ca2+ and the activities of protein tyrosine phosphatases, in particular SHP-1, are elevated.