Gary B. Huffnagle, Ph.D.
Professor,
Department of Internal Medicine (Pulmonary) and
Department of Microbiology and Immunology

ghuff@umich.edu

Overall, the focus of our laboratory is on the concept of "Homeostasis Thresholds" in the immune system, with emphasis on immune responses in the lungs and sinuses to fungal (Cryptococcus, Aspergillus) or allergen challenge. Homeostasis means physiologic balance. Threshold refers to the cross-over point in an immunologic challenge at which the balance between anti-inflammatory and inflammatory mechanisms shifts to invoke an inflammatory process to handle the challenge. Below the threshold, anti-inflammatory processes predominate.Prevents complete elimination of a microbe due to a balance between inflammatory and anti-inflammatory mechanisms. The important features of this concept is that Homeostasis Threshold is 1) an in vivo response not predicted by in vitro biology, 2) due to the ability of host defenses to amplify and perpetuate inflammatory signals (in other words, just because the antigen is gone does not mean a response will stop - the response has to be shut down), 3) a response that prevents damage to the host and promotes antigen retention for immunologic memory and 4) is the underlying principle of the “damage-response framework” theory proposed by Casadevall and Pirofski.

Increasingly, epidemiologic and clinical data support the hypothesis that perturbations in the gastrointestinal microbiota due to antibiotic use and dietary differences in “industrialized” countries have disrupted the normal microbiota-mediated mechanisms of immunological tolerance in the mucosa, leading to an increase in the incidence of allergic airway disease. The data supporting this "Microflora Hypothesis" includes correlations between allergic airway disease and (1) antibiotic use early in life, (2) altered fecal microbiota and (3) dietary changes over the past two decades. Our laboratory has demonstrated that mice can develop allergic airway responses to allergens if their endogenous microbiota is altered at the time of first allergen exposure. These experimental and clinical observations are consistent with other studies demonstrating that the endogenous microbiota plays a significant role in shaping the development of the immune system. Data is beginning to accumulate that a "balanced" microbiota plays a positive role in maintaining mucosal immunologic tolerance long after post-natal development. Other studies have demonstrated that even small volumes delivered to the nasopharynx largely end up in the GI tract, suggesting that airway tolerance and oral tolerance may operate simultaneously. The mechanism of microbiota modulation of host immunity is not known; however, host and microbial oxylipins are one potential set of immunomodulatory molecules that may control mucosal tolerance. Probiotic and prebiotic immunotherapy may work through modulating the ecology of the GI microbiota.

Cryptococcus neoformans is an environmental fungus that has developed an array of mechanisms to cope with challenges in the environment such as competing microbes, amoeba, UV radiation, temperature and plant defenses. As it turns out, these "survival" mechanisms become "virulence" factors when C. neoformans infects a host. In turn, most hosts possess a broad spectrum of defense mechanisms that limit the pathogenesis of C. neoformans. In mammals, these include both innate and adaptive immune responses. However, despite a multi-factoral defense system, the end result of host defense against C. neoformans appears to be control but not clearance of the organism. Small numbers of yeast appear to persist, potentially for the life of the host, and these yeast can serve as a nidus for reactivation disease should the host become immunocompromised.

We have been interested in the immunological dynamics of this response. On the microbial side of the equation, C. neoformans can prevent the generation of early inflammatory signals following infection. The organism can also produce a number of oxylipins (oxygenated fatty acid metabolites) that have the ability to down-modulate immune responses. The role of oxylipins in pathogenesis is a focus of current investigations in the lab. TNF is an early inflammatory mediator whose production is critical for the development of protective T1 type immunity to C. neoformans. The target of TNF activity appears to be in driving the maturation of dendritic cells as they migrate from the site of infection into draining lymph nodes. A transient deficiency of TNF at the onset of infection can result in an aberrant programming of the T cell response such that a chronic infection ensues that is not cleared even when TNF levels return to normal levels. One of the important questions about the development of protective T cell immunity in the lungs is where does T cell polarization occur, in the draining lymph nodes or at the site of infection? Our recent studies suggest that CD4 T cell polarization is largely controlled locally. Should T cell polarization drift toward T2, instead of T1, the result will be the development of alternatively activated macrophages that cannot clear the yeast and can even contribute to disease pathology. In animal models, the most striking feature of a T2 response to C. neoformans in the lungs is the accumulation of YM1 crystals (a chitinase-like molecule) in macrophages.

Perhaps the most striking feature of the host-cryptococcus dynamic during infection is the observation that mice can harbor very low numbers of yeast without clearing them but, when challenged with another bolus of cryptococci, can very efficiently control and clear the second infection. Thus, persistent low-grade infections present an immunological paradox that challenges the notion that the function of the immune system is to clear infectious agents. Rather, it appears that the function of the immune system is to provide a homeostatic balance with the microbial world around us and protect the other systems in the body (respiratory, nervous, GI, etc.) from damage. Therefore, low level infection that does not cause damage will be tolerated and may even be important for immune functioning by serving as a source of antigen for "memory" responses.

Representative Publications

Noverr, M. C., N. R. Falkowski, R. A. McDonald, A. N. McKenzie, and G. B. Huffnagle. 2005. Development of allergic airway disease in mice following antibiotic therapy and fungal microbiota increase: role of host genetics, antigen, and interleukin-13. Infect Immun 73:30.

Lindell, D. M., T. A. Moore, R. A. McDonald, G. B. Toews, and G. B. Huffnagle. 2005. Generation of Antifungal Effector CD8+ T Cells in the Absence of CD4+ T Cells during Cryptococcus neoformans Infection. J Immunol 174:7920.

Herring, A. C., N. R. Falkowski, G. H. Chen, R. A. McDonald, G. B. Toews, and G. B. Huffnagle. 2005. Transient neutralization of tumor necrosis factor alpha can produce a chronic fungal infection in an immunocompetent host: potential role of immature dendritic cells. Infect Immun 73:39.

Hernandez, Y., S. Arora, J. R. Erb-Downward, R. A. McDonald, G. B. Toews, and G. B. Huffnagle. 2005. Distinct roles for IL-4 and IL-10 in regulating T2 immunity during allergic bronchopulmonary mycosis. J Immunol 174:1027.

Arora, S., Y. Hernandez, J. R. Erb-Downward, R. A. McDonald, G. B. Toews, and G. B. Huffnagle. 2005. Role of IFN-gamma in regulating T2 immunity and the development of alternatively activated macrophages during allergic bronchopulmonary mycosis. J Immunol 174:6346.

Olszewski, M. A., M. C. Noverr, G. H. Chen, G. B. Toews, G. M. Cox, J. R. Perfect, and G. B. Huffnagle. 2004. Urease expression by Cryptococcus neoformans promotes microvascular sequestration, thereby enhancing central nervous system invasion. Am J Pathol 164:1761.

Noverr, M. C., R. M. Noggle, G. B. Toews, and G. B. Huffnagle. 2004. Role of antibiotics and fungal microbiota in driving pulmonary allergic responses. Infect Immun 72:4996.

Noverr, M. C., and G. B. Huffnagle. 2004. Regulation of Candida albicans morphogenesis by fatty acid metabolites. Infect Immun 72:6206.

Noverr, M. C., and G. B. Huffnagle. 2004. Does the microbiota regulate immune responses outside the gut? Trends Microbiol 12:562.

.