Serotonin transporter protein overexpression and association to Th 17 and T regulatory cells in lupoid leishmaniasis

The immunopathogenesis of chronic non-healing Old World cutaneous leishmaniasis is challenging. There is a bidirectional communication between the nervous and immune systems, serotonin being an important mediator in this respect. Our aim was to study the role of the serotonin transporter protein (SERT) and its relation to T cell-related immune responses in lupoid leishmaniasis. Paraffin-embedded skin biopsies of 12 cases of lupoid and 12 cases of usual types of cutaneous leishmaniasis were investigated using immunohistochemistry regarding expression of SERT, Th1, Th2, Th17 and T regulatory cell (Treg) markers. SERT as well as Tregs and interleukin (IL)-17 positive cells were more prevalent while IL-5 (Th2) and interferon (IFN)-γ (Th1) expressing cells were less numerous in the lupoid tissue compared to those from the usual type of leishmaniasis. The majority of the SERT⁺ cells were also tryptase⁺ (mast cells). There was a positive correlation between a higher number of SERT⁺ and IL-17⁺ cells in the lupoid type, while lower numbers of SERT⁺ cells were significantly related to lower percentages of CD25⁺ cells in the usual type of leishmaniasis. These results might indicate a role for SERT, Th17 and Tregs in the pathogenesis of lupoid leishmaniasis.