THE ROLE OF COMPLEMENT IN THE IMMUNE RESPONSE AGAINST CRYPTOCOCCAL INFECTION

Abstract

Cryptococcus neoformans is an encapsulated fungal pathogen with ubiquitous environmental contribution. After being inhaled, the fungus primarily proliferates in the lungs but can migrate to the bloodstream and cause disseminated infections in other organs mainly in immunocompromised individuals. Once in the bloodstream, the fungus can cross the blood-brain barrier and establish cranial infection, leading to meningoencephalitis. Cryptococcal meningoencephalitis is a leading cause of mortality in HIV/AIDS patients, accounting for 112,000 deaths each year. The host complement system plays a prominent role in innate and adaptive immune responses to microbial infections. In clinical practice, complement blockade poses a high risk for disseminated cryptococcosis. However, the role of complement, especially complement receptors, in host defense against cryptococcal infection remains incompletely understood. In this study, we used both intratracheal and intravenous infection models to examine how genetic ablation of the central complement protein C3 and complement receptors including C3aR, C5aR1, and C5aR2 affects the cryptococcal pathogenesis and the cellular immune responses to C. neoformans infection. The intratracheal infection model mimics pulmonary infection of C. neoformans in natural scenario and represents immune response against cryptococcal infection in the lung and dissemination into other organs. The intravenous infection model mimics an invasive systemic infection model and represents the ability of intravascular killing and fungal proliferation in different organs. In the intratracheal infection model, we examined the role of C5aR1 signaling in host defense against C. neoformans infection in the lung. The results showed that C5aR1-/- mice exhibited significantly higher fungal burdens in the lung at 6 weeks post-infection as compared to wild-type mice. Fungal burdens in other organs of C5aR1-/- mice, including spleen, kidney, and brain were also higher at 6 weeks post infection. Deficiency of C5aR1 resulted in significantly lower number of leukocytes in the lung and reducing the number of alveolar macrophages, compared to wild-type mice at 6 weeks after intratracheal infection. Moreover, the reduction of alveolar macrophages was associated with enhanced fungal burdens in the bronchoalveolar lavage fluid of C5aR1-/- mice. Thus, genetic ablation of C5aR1 resulted in impaired fungal clearance in the lung following airway infection with C. neoformans, associated with reduced number of alveolar macrophages. In the intravenous infection model, we found that deficiency of C3 and C5aR1, but not C3aR or C5aR2, resulted in uncontrolled fungal growth in the lung, eventually leading to early death of the infected mice. Infected C3-/- and C5aR1-/- mice displayed higher accumulation of leukocytes including Ly6Chi inflammatory monocytes, interstitial macrophages, CD4+ T cells, and CD8+ T cells with reduction of the frequency and number of alveolar macrophages in the lung, compared to infected wild-type mice. Deficiency of C3 and C5aR1 also led to enhanced secretions of inflammatory cytokines including IFN-γ and IL-1β. Taken together, this study demonstrated that C3 and C5aR1 contribute to fungal clearance in the lung and host survival during systemic infection with C. neoformans.