Antibody-Mediated Protection in MurineCryptococcus neoformansInfection Is Associated with Pleotrophic Effects on Cytokine and Leukocyte Responses

Abstract

Cryptococcus neoformans, an encapsulated yeast, is a common cause of life-threatening meningoencephalitis in immunosuppressed patients. We previously observed that administration of a monoclonal antibody (MAb) to the capsular polysaccharide to mice with pulmonary infection prolonged survival and enhanced granulomatous inflammation without reducing lung CFU. To understand the mechanism of MAb action, we studied leukocyte recruitment and cytokine profiles in lungs of A/JCr mice. B lymphocytes were the predominant cell type in lung infiltrates, comprising 15 to 30% of the leukocytes. Despite alterations in histological appearance, fluorescence-activated cell sorter analysis revealed no significant difference in total numbers of lung leukocytes in MAb-treated mice and controls. Differences in the immune response to C. neoformans between MAb-treated mice and controls included (i) an increase in the percentage of granulocytes among lung leukocytes on day 14, (ii) higher macrophage surface expression of CD86 on day 28, (iii) larger amounts of IL-10 in lung homogenates at day 7, (iv) a trend toward smaller amounts of gamma interferon mRNA and protein on day 7, and (v) a smaller increase in the levels of interleukin-4 mRNA and protein on day 7. Hence, the immune responses to C. neoformans infection in the presence and absence of specific antibody were qualitatively similar, and antibody administration was associated with several subtle quantitative differences in immune response parameters that could translate into enhanced survival. MAb may function partly by down-regulating the inflammatory response and reducing host damage. Our findings demonstrate unexpected complexity in the interaction between specific MAb and other components of the host immune response.Cryptococcus neoformans is an encapsulated fungus that is both an intracellular and an extracellular pathogen (22, 34). The medical importance of this organism has increased in the past two decades as a consequence of the AIDS epidemic, with the incidence approaching 30% in patients with AIDS in parts of sub-Saharan Africa (14). The lung is the portal of entry of C. neoformans in humans. In immunocompetent hosts, infection is presumably contained in the lung and the majority of human infections are asymptomatic. However, in patients with impaired immunity, extrapulmonary infection is common (35). Currently available antifungal therapies do not eradicate infection in the severely immunocompromised host, and there is considerable interest in the development of both immunomodulatory therapy and an effective vaccine (12, 23). Toward this end, a phase I trial of a monoclonal antibody (MAb) that binds to the glucuronoxylomannan (GXM) component of the capsular polysaccharide (9) is nearing completion.Pulmonary defense mechanisms against C. neoformans have been studied extensively in murine experimental infection (29,33,38). In response to C. neoformans infection, mice develop a heterogeneous tissue response that ranges from granuloma formation with intracellular yeast cel...

Microbial Immune Suppression Mediated by Direct Engagement of Inhibitory Fc Receptor

“…Our studies, as well as others, demonstrated that the GXM-mAb complex counteracts the suppression exerted by GXM (20,37,42). This suggests an essential role for macrophages in acquiring and storing GXM in various organs and is consistent with previous observations of GXM accumulation inside tissue macrophages in vivo (43,44).…”

“…There are multiple mechanisms by which protective Abs to GXM can induce changes in the immune response that can translate into improved host responses. In particular, the interdependency between humoral and cellular-mediated immunity and the mechanisms through which Abs to GXM regulate cell-mediated immunity have been demonstrated previously (20,23,24). Our studies have shown that mouse mAbs to GXM can reverse the negative regulation exerted by GXM 1) by inducing secretion of proinflammatory cytokines, such as IL-1␤, IL-12, and TNF-␣, 2) by reducing production of IL-10 (10, 25, 26), 3) by promoting expression of costimulatory molecules on monocytes/macrophages, such as B7-1 (1) that are usually suppressed by presence of capsular material (27), and 4) by increasing phagocytosis, enhancing killing activity, and restoring IL-8 released from neutrophils of AIDS patients (28,29).…”

Mouse Genetic Background Is a Major Determinant of Isotype-Related Differences for Antibody-Mediated Protective Efficacy againstCryptococcus neoformans

“…mAb IgG3-treated mice had lower levels of IFN-␥ and higher levels of IL-2 and IL-4. Although the mechanism for this effect is not understood, Abmediated protection has been associated with lower levels of IFN-␥ in A/JCr mice (17). Although Th1 responses are critical for an effective host response against C. neoformans in mice (33,34,37,56,57), Th2-related cytokines are also necessary for Ab function (16).…”

“…For example, Ab-mediated protection against C. neoformans depends on Ab specificity (11), Ab isotype (10,12), T cell function (9), the C. neoformans strain studied (13), Ab quantity (14), expression of inducible NO synthase 2 (NOS2) 4 (15), and the presence of Th1-and Th2-related cytokines (16). Furthermore, Abmediated protection against murine pulmonary C. neoformans infection is accompanied by changes in the inflammatory response that reflect differences in cellular infiltrate and cytokine response (15,17).…”

Passive Immunization of Neonatal Mice againstPneumocystis cariniif. sp.murisEnhances Control of Infection without Stimulating Inflammation

“…However, the lack of inflammation in mice with passive antibody is consistent with the notion that antibody responses can either enhance or reduce inflammation depending on the interactions with cell-mediated immunity and with the organisms (6). Passive antibody has been shown to decrease airway inflammation in a murine model of respiratory syncytial virus infection (28) and prolong survival of mice infected with Cryptococcus neoformans by changing the characteristics and intensity of the pulmonary cellular and cytokine response (7). One explanation for the lack of generation of an inflammatory response to Pneumocystis in neonatal mice might be that there is likely transforming growth factor ␤ present in the postnatally developing lungs (33), which has been shown to inhibit Fc␥ receptor signaling in murine macrophages (35).…”