Expression of T-cell-associated serine proteinase 1 during murine Leishmania major infection correlates with susceptibility to disease

CD8+ T cells in cutaneous leishmaniasis: the good, the bad, and the ugly

CD8(+) T lymphocytes are components of the adaptive immune response and play an important role in protection against many viral and bacterial infections. However, their role in parasitic infections is less well understood. In leishmaniasis, a disease caused by intracellular protozoan parasites of the genus Leishmania, CD8(+) T cells have been shown to be protective. However, increasing evidence indicates that CD8(+) T cells may also exacerbate disease. In this review, we will describe the situations where CD8(+) T cells are either good or bad for the outcome of the infection and attempt to reconcile the dual role played by CD8(+) T cells in cutaneous leishmaniasis.

Recruitment of CD8(+) T cells expressing granzyme A is associated with lesion progression in human cutaneous leishmaniasis

Human infection with Leishmania braziliensis leads to the establishment of cutaneous leishmaniasis (CL), characterized by the appearance of skin lesions that progress from nonulcerated to ulcerated forms. Our goal was to characterize the immunological kinetics associated with this progression, comparing the cellular composition, cytokines and granzyme expression between lesions of patients with early (E-CL) and late stages (L-CL) of CL. Histopathological analysis showed that lesions from L-CL had more exuberant inflammatory infiltrate as compared to E-CL. Although E-CL and L-CL lesions were predominantly mononuclear, lesions from E-CL patients presented higher neutrophil and eosinophil counts than L-CL. While percentages of CD4(+) and of CD68(+) cells were slightly higher in L-CL, a fivefold increase of CD8(+) cells was observed in L-CL, as compared to E-CL. Moreover, CD8(+) T-cells from L-CL expressed significantly higher levels of granzyme A than E-CL. Interestingly, granzyme A expression was positively correlated with intensity of the inflammatory infiltrate in L-CL but not E-CL. Lastly, percentages of IFN-gamma(+) and IL-10(+) cells were higher in L-CL as compared to E-CL, with CD4(+) T-cells and CD68(+) monocytes as the main sources of these cytokines, respectively. These results suggest that recruitment of CD8(+) granzyme A(+) T cells is involved in lesion progression in human CL.

Dual mechanisms of apoptosis induction by cytotoxic lymphocytes

Cytotoxic T lymphocytes and natural killer cells together comprise the means by which the immune system detects and rids higher organisms of virus-infected or transformed cells. Although differing considerably in the way they detect foreign or mutated antigens, these cells utilize highly analogous mechanisms for inducing target cell death. Both types of effector lymphocytes utilize two principal contact-dependent cytolytic mechanisms. The first of these, the granule exocytosis mechanism, depends on the synergy of a calcium-dependent pore-forming protein, perforin, and a battery of proteases (granzymes), and it results in penetration by effector molecules into the target cell cytoplasm and nucleus. The second, which requires binding of FasL (CD95L) on the effector cell with trimeric Fas (CD95) molecules on receptive target cells, is calcium independent and functions by generating a death signal at the inner leaflet of the target cell membrane. Exciting recent developments have indicated that both cytolytic mechanisms impinge on an endogenous signaling pathway that is strongly conserved in species as diverse as helminths and humans and dictates the death or survival of all cells.

Cutaneous leishmaniasis: co-ordinate expression of granzyme A and lymphokines by CD4+ T cells from susceptible mice

We have recently demonstrated that the frequency of T cells expressing granzyme A is significantly higher in skin lesions and spleens of susceptible BALB/c mice compared with resistant C57BL/6 mice infected with Leishmania major, a cause of human cutaneous leishmaniasis. In the present study, we have performed in vitro studies to characterize the subpopulation, the antigen responsiveness and the lymphokine production pattern of granzyme A-expressing T cells in L. major-infected mice. Using a limiting dilution system for functional analysis of selected T cells at the clonal level, we could show that granzyme A activity in infected BALB/c mice can be assigned to L. major-reactive CD4+ T cells secreting interleukin-2 (IL-2) and IL-4. Granzyme A production was most pronounced in the early phase of infection. On the other hand, granzyme A expression could not be detected in C57BL/6-derived T cells responding to L. major. The data support the suggestion that granzyme A is produced by L. major-responsive CD4+ T cells facilitating lesion formation and the dissemination of infection.