Date Published: March 2, 2016
Publisher: Public Library of Science
Author(s): Anshu P. Gounder, Nicolle D. Myers, Piper M. Treuting, Beth A. Bromme, Sarah S. Wilson, Mayim E. Wiens, Wuyuan Lu, André J. Ouellette, Katherine R. Spindler, William C. Parks, Jason G. Smith, William J Britt.
α-defensins are abundant antimicrobial peptides with broad, potent antibacterial, antifungal, and antiviral activities in vitro. Although their contribution to host defense against bacteria in vivo has been demonstrated, comparable studies of their antiviral activity in vivo are lacking. Using a mouse model deficient in activated α-defensins in the small intestine, we show that Paneth cell α-defensins protect mice from oral infection by a pathogenic virus, mouse adenovirus 1 (MAdV-1). Survival differences between mouse genotypes are lost upon parenteral MAdV-1 infection, strongly implicating a role for intestinal defenses in attenuating pathogenesis. Although differences in α-defensin expression impact the composition of the ileal commensal bacterial population, depletion studies using broad-spectrum antibiotics revealed no effect of the microbiota on α-defensin-dependent viral pathogenesis. Moreover, despite the sensitivity of MAdV-1 infection to α-defensin neutralization in cell culture, we observed no barrier effect due to Paneth cell α-defensin activation on the kinetics and magnitude of MAdV-1 dissemination to the brain. Rather, a protective neutralizing antibody response was delayed in the absence of α-defensins. This effect was specific to oral viral infection, because antibody responses to parenteral or mucosal ovalbumin exposure were not affected by α-defensin deficiency. Thus, α-defensins play an important role as adjuvants in antiviral immunity in vivo that is distinct from their direct antiviral activity observed in cell culture.
In addition to a sophisticated adaptive immune system, mammals retain more primitive immune effectors, such as antimicrobial peptides, as components of the innate response to microbial infection. In humans, one of the most abundant classes of antimicrobial peptides is α-defensins [1, 2]. α-defensins are subdivided into myeloid α-defensins [e.g., human neutrophil peptides (HNP) 1–4], expressed primarily in neutrophils and certain other immune cells, and enteric α-defensins [e.g., human defensins (HD) 5 and 6], expressed by specialized Paneth cells in the small intestinal epithelium and by epithelial cells in the genitourinary tract. α-defensins have potent antiviral and antibacterial activities in vitro and in cell culture against a wide range of organisms. Although the multifaceted contribution of α-defensins to shaping the composition of the ileal bacterial commensal microbiota and to defense against multiple enteric bacterial pathogens in vivo has been described, comparable studies of α-defensin antiviral activity in vivo are lacking . Moreover, clinical correlations between defensin abundance and viral transmission or disease are not clear . To address this gap in knowledge, we investigated mouse adenovirus type 1 (MAdV-1) pathogenesis in mice lacking functional enteric α-defensin processing, the matrix metalloproteinase-7 knockout (Mmp7-/-) mouse, as a system in which to study a viral pathogen in its natural host.
The impact of α-defensins on viral transmission or pathogenesis has not been previously examined experimentally; however, correlates from clinical samples suggest that high α-defensin levels are associated with reduced transmission of HIV-1 and slower disease progression . In studies of β- and θ-defensins in viral pathogenesis, the expression or administration of defensin reduced viral immunopathology without impacting viral titers [31, 32]. These studies suggest a more profound effect of defensins on limiting innate immunopathology than as direct antivirals. Similarly, despite their direct antiviral activity in cell culture, the effect of α-defensins on MAdV-1 pathogenesis appears to be indirect. Taken together, our data support a mechanism in which functional processing of enteric α-defensins at the initial site of viral infection in the small intestine is a critical modulator of the protective NAb response, which is required for survival from acute MAdV-1 infection . Myeloid α-defensins (HNPs) function as adjuvants [30, 33]. When mixed with OVA and administered intranasally, HNPs increased anti-OVA serum IgG but not IgA . HNPs also enhanced CD4+ T cell cytokine secretion and proliferation following stimulation either in vivo or in vitro, suggesting an ability of HNPs to stimulate T cell-dependent cellular and humoral immunity. This was substantiated in intraperitoneal immunization studies with other model antigens and tumor-specific antigens . Enteric α-defensins, either mouse or human, have not been previously reported as adjuvants. Moreover, this is the first example of a direct role for an α-defensin in engendering an adaptive response to a pathogen.