Immunopathogenic mechanisms of non-healing cutaneous leishmaniasis caused by Leishmania amazonensis infection
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The objective of my dissertation project is to understand mechanisms employed by Leishmania amazonensis to evade host immune components, particularly B cells and interferon-gamma. Our data have shown that L. amazonensis infection induces B cell activation, leading to enhanced lesion development. In the absence of B cells, L. amazonensis-infected mice developed a delayed onset of disease, correlating with an impaired activation of antigen-specific pathogenic CD4+ T cells. Flow cytometry and immunohistochemical studies suggest that B cells may provide secondary signals for activating T helper cells and recruiting leukocytes to the site of infection. The pathogenic role of B cells was suggested to be partly due to the presence of antibodies, which induced dendritic cell activation and promoted efficient priming of parasite-specific T cells to produce high levels of IL-10. The investigation of the role of IFN-g in L. amazonensis infection revealed that IFN-g alone could promote parasite growth within macrophages (MFs) by inducing the expression of mouse cationic amino acid transporter-2B (mCAT-2B), a key transporter for L-arginine. Although a clear upregulation of mCAT-2B and L-arginine transport was detected, no measurable iNOS or arginase activities were observed in IFN-g-treated, infected MFs. These data suggest an involvement of a novel L-arginine usage independent of iNOS and arginase activities during IFN-g-mediated parasite growth enhancement. The ability to scavenge available L-arginine directly by the parasites is proposed. In addition to inducing pathogenic B cells and interfering with L-arginine pathways, these parasites were found to express molecules similar to the mammalian chemokine receptor, CCR5. Although the function of a parasite-derived CCR5-like molecule remains unclear, it may help the parasites to find immature “safe target” cells. Together, this project provides new insights into the complex interactions between the parasite and host immune system and indicates the challenges we will face in the control of non-healing cutaneous leishmaniasis.