Optimal CD8 T-Cell Response against Encephalitozoon cuniculi Is Mediated by Toll-Like Receptor 4 Upregulation by Dendritic Cells

CD8⁺ T-cell immunity has been shown to play an important role in the protective immune response against Encephalitozoon cuniculi. Although earlier studies suggest that dendritic cells (DC) are important for the induction of this response, the factors responsible for initiation of the dendritic cell response against this pathogen have not been evaluated. In the current study, we demonstrate that E. cuniculi infection causes strong Toll-like receptor 4 (TLR4)-dependent dendritic cell activation and a blockade of this molecule reduces the ability of DC to prime an antigen-specific CD8⁺ T-cell response. Pretreatment of DC with anti-TLR4 antibody causes a defect in both in vitro and in vivo CD8⁺ T-cell priming. These findings, for the first time, emphasize the contribution of TLR4 in the induction of CD8⁺ T-cell immunity against E. cuniculi infection.Microsporidia are small obligate intracellular parasites that, until recently, were thought to be protozoans; however, evidence now suggests that they are related to fungi (15, 17). Microsporidia can infect a vast number of species of vertebrates and invertebrates; of the 150 genera of microsporidia, however, only 7 have been found to infect humans (13). Severe infections have been reported predominantly for immunocompromised patients, such as patients with HIV and organ transplant recipients (2, 7, 23, 37). Acute infections have also been reported in travelers and the elderly (26, 27), and there is evidence of colonization of healthy, nonsymptomatic patients (34).Due to the prevalence of opportunistic microsporidian infections associated with the HIV-AIDS pandemic, recent research has focused on the host''s immune response to these pathogens. Early animal studies showed that cellular immunity was necessary to protect SCID mice from a lethal Encephalitozoon cuniculi challenge. Moreover, depletion of CD8⁺ T cells caused mice to succumb to intraperitoneal (i.p.) E. cuniculi infection (21), and previous studies in our laboratory have shown that cytotoxic lymphocytes play a major role in protection against this effect (20, 21).Recent reports from our laboratory have demonstrated that dendritic cells (DC) play an important role in the priming of the immune response against E. cuniculi (31, 32). T cells incubated with E. cuniculi-pulsed DC exhibited antigen-specific characteristics, specifically gamma interferon (IFN-γ) production, cytotoxicity, and proliferation (31, 32). In order to mount an immune response against a foreign pathogen, DC must first recognize the pathogen to initiate an appropriate response. One key method of recognition is through Toll-like receptors (TLRs), which were first discovered in Drosophila in response to infection with fungal pathogens (24). However, specific TLR molecules involved in DC activation during E. cuniculi infection have not been identified previously. We evaluated the upregulation of specific molecules involved in activation of the DC response after E. cuniculi infection. Different TLR molecules were tested, and TLR4 expression was found to be essential for induction of the optimal CD8⁺ T-cell response by these cells.