Role of Dendritic Cells and Alveolar Macrophages in Regulating Early Host Defense against Pulmonary Infection with Cryptococcus neoformans

Successful pulmonary clearance of the encapsulated yeast Cryptococcus neoformans requires a T1 adaptive immune response. This response takes up to 3 weeks to fully develop. The role of the initial, innate immune response against the organism is uncertain. In this study, an established model of diphtheria toxin-mediated depletion of resident pulmonary dendritic cells (DC) and alveolar macrophages (AM) was used to assess the contribution of these cells to the initial host response against cryptococcal infection. The results demonstrate that depletion of DC and AM one day prior to infection results in rapid clinical deterioration and death of mice within 6 days postinfection; this effect was not observed in infected groups of control mice not depleted of DC and AM. Depletion did not alter the microbial burden or total leukocyte recruitment in the lung. Mortality (in mice depleted of DC and AM) was associated with increased neutrophil and B-cell accumulation accompanied by histopathologic evidence of suppurative neutrophilic bronchopneumonia, cyst formation, and alveolar damage. Collectively, these data define an important role for DC and AM in regulating the initial innate immune response following pulmonary infection with C. neoformans. These findings provide important insight into the cellular mechanisms which coordinate early host defense against an invasive fungal pathogen in the lung.Cryptococcus neoformans, an opportunistic fungal pathogen acquired through inhalation, causes significant morbidity and mortality primarily in patients with impairments in host defense, including those with AIDS, those with lymphoid or hematological malignancies, or those receiving immunosuppressive therapy secondary to autoimmune disease or organ transplantation (31, 33, 60). The development of a T1 antigen-specific immune response characterized by gamma interferon production and classical activation of macrophages is required to eradicate the organism (4, 8, 21, 23, 24, 28). This adaptive immune response takes 2 to 3 weeks to develop and coincides with the CCR2-mediated recruitment of additional pulmonary dendritic cells (DC) and T cells to the lung (51, 66, 67). The role of initial, innate immune responses against the organism (prior to the development of adaptive immunity) is not well understood.Resident lung phagocytic cells, primarily DC and alveolar macrophages (AM), are likely the first immune cells exposed to C. neoformans upon inhalation of the organism into the lung. Both DC and AM express lectin receptors, including macrophage mannose receptor and DC-specific non-ICAM3 grabbing nonintergrin (DC-SIGN) (14, 15), which bind C. neoformans glycoantigens, including mannoproteins (42, 55). DC and AM phagocytose the organism in vitro and in vivo (29, 34, 63, 77, 78), and phagocytosis (and/or exposure to soluble glycoantigens or cryptococcal DNA) is associated with cytokine and chemokine production (5, 29, 40, 48, 49, 55, 61) and yeast lysis (77). It is unclear whether phagocytosis by resident DC and AM contributes to early clearance and/or the later development of adaptive immunity.DC represent an important interface between innate and adaptive immunity (reviewed in references 25, 53, 59, and 62). DC-cryptococcal interactions alter DC antigen-presenting functions and modulate resultant T-cell responses in vitro (10, 19, 55, 71). Following cryptococcal infection in vivo, DC migrate to thoracic lymph nodes (4, 52, 67). Thereafter, newly recruited DC colocalize with T cells within bronchovascular infiltrates in the lung (51). This is associated with interleukin-12 and gamma interferon production, yet direct evidence that DC-T-cell interactions modulate anticryptococcal responses in vivo remains sparse.A well-described murine model of in vivo DC depletion has been used to evaluate the contribution of DC to the development of dynamic, antigen-specific immune responses against a variety of antigens and microbial pathogens (16, 26, 27, 37, 56, 57, 69). Depletion results from the administration of diphtheria toxin (DT) to transgenic (Tg) mice in which the DT receptor (DTR) has been linked to the CD11c promoter. DT administration transiently depletes tissue DC (which express CD11c) for up to 96 h. This model has helped in assessment of the role of pulmonary DC in mediating adaptive immune responses against inhaled antigen (ovalbumin) and influenza virus (16, 69). In these studies, DT administration was associated with transient depletion of AM (which also express CD11c). The effect of CD11c cell depletion on innate immune responses has not been assessed (or reported) in these studies.In the current study, this established model of DT-mediated depletion of resident DC and AM was used to assess the in vivo contribution of these cells to the initial host response against cryptococcal infection. This objective is clinically relevant, as studies modulating DC numbers are in development for the treatment of patients with asthma (30, 32), autoimmunity (65, 68, 74), organ transplantation (12, 39, 43), and cancer (3, 11, 73). It is unknown whether manipulating the number of tissue DC will alter the innate or adaptive antifungal host defense in these patients. Our results demonstrate that DC and AM are critical regulators of the initial immune response against C. neoformans within the lung. Early mortality in DC- and AM-depleted mice precluded our ability to assess the role of these cells in the development of adaptive immunity.