Date Published: November 20, 2014
Publisher: Public Library of Science
Author(s): Ifeoma B. Okwor, Ping Jia, Zhirong Mou, Chukwunonso Onyilagha, Jude E. Uzonna, Laurence U. Buxbaum. http://doi.org/10.1371/journal.pntd.0003300
Abstract: We previously showed that CD8+ T cells are required for optimal primary immunity to low dose Leishmania major infection. However, it is not known whether immunity induced by low dose infection is durable and whether CD8+ T cells contribute to secondary immunity following recovery from low dose infection. Here, we compared primary and secondary immunity to low and high dose L. major infections and assessed the influence of infectious dose on the quality and magnitude of secondary anti-Leishmania immunity. In addition, we investigated the contribution of CD8+ T cells in secondary anti-Leishmania immunity following recovery from low and high dose infections. We found that the early immune response to low and high dose infections were strikingly different: while low dose infection preferentially induced proliferation and effector cytokine production by CD8+ T cells, high dose infection predominantly induced proliferation and cytokine production by CD4+ T cells. This differential activation of CD4+ and CD8+ T cells by high and low dose infections respectively, was imprinted during in vitro and in vivo recall responses in healed mice. Both low and high dose-infected mice displayed strong infection-induced immunity and were protected against secondary L. major challenge. While depletion of CD4+ cells in mice that healed low and high dose infections abolished resistance to secondary challenge, depletion of CD8+ cells had no effect. Collectively, our results show that although CD8+ T cells are preferentially activated and may contribute to optimal primary anti-Leishmania immunity following low dose infection, they are completely dispensable during secondary immunity.
Partial Text: The spectrum of disease collectively known as leishmaniasis continues to be a major threat to global health in many regions of the world. According to the World Health Organization (WHO) estimate, about 15–20 million people are afflicted with the disease and close to 2 million new cases occur annually . Despite intensive research, there is currently no effective licensed vaccine for prevention of human leishmaniasis. This is in part related to lack of proper understanding of the immunobiology of the disease, particularly the factors that regulate the induction, maintenance and loss of protective immunity. Because Leishmania are obligate intracellular parasites, a strong T cell-mediated immunity is critical for effective control of the infection. Indeed, T cell deficient mice are highly susceptible to Leishmania infection, and adoptive transfer of T cells restores resistance in these mice . Although it is widely believed that CD4+ T cells are the key lymphocyte subset that regulates anti-Leishmania immunity, studies utilizing low dose infections show that CD8+ T cells are also important for optimal primary immunity , . Thus, while CD8 deficient mice are still resistant to high dose L. major infection, low dose infection of these mice results in uncontrolled parasite proliferation and impaired IFN-γ response . However, no study has addressed the impact of parasite dose on the magnitude of initial T cell (both CD4+ and CD8+) expansion, and whether this affects the development of secondary anti-Leishmania immunity. In addition, the contribution of CD8+ T cells to low dose infection-induced resistance is not known.
Clinical observations and experimental studies suggest that the development of effective cell-mediated immunity is essential for protection against leishmaniasis. However, the lack of a universally approved and effective vaccine against human leishmaniasis suggests that we still do not completely understand the factors that regulate the development of cell-mediated immunity against the disease. Although CD4+ T cells are critical for protective immunity in cutaneous leishmaniasis , CD8+ T cells have also been shown to be essential in certain situations, particularly in low dose infections , . During low dose L. major infection, CD8+ T cells were shown to contribute to lesion resolution and parasite control by producing IFN-γ that augment optimal CD4+ Th1 response , . However, whether CD8+ T cells also contribute to secondary anti-Leishmania immunity following resolution of primary infection is unclear. In addition, no study has investigated the impact of low parasite dose on secondary anti-Leishmania immunity. We show here that the patterns of lesion development and parasite burden were similar in both high and low dose infections, the quality of the immune response was strikingly different. Whereas high dose infection induced strong CD4+ T cell proliferation and Th1 cytokine response, low dose infection predominantly activates CD8+ T cells. Using adoptive transfer of T cells from healed mice into naïve congenic recipients, we also demonstrated that this differential activation of CD4+ and CD8+ T cells by high and low dose infections, respectively, is observed in vivo following L. major challenge. Interestingly, while depletion of CD4+ T cells in mice that healed both high and low dose infections led to loss of immunity following secondary L. major challenge, depletion of CD8+ T cells had no effect. Taken together, the results presented here show that although low dose L. major infection preferentially activates CD8+ T cells that contribute to optimal primary immunity, they are completely dispensable for resolution of secondary L. major challenge.