Immune mechanisms of pathogenetic synergy in concurrent bovine pulmonary infection with Haemophilus somnus and bovine respiratory syncytial virus

Abstract

Bovine respiratory syncytial virus (BRSV) and Haemophilus somnus are two bovine respiratory pathogens that cause disease singly or as part of a polymicrobial infection. BRSV infection is often associated with a predisposition towards production of a T helper type 2 (Th2) response and IgE production. In contrast, an IgG2 response to H. somnus has been shown to be most important for recovery. An experiment was performed to evaluate the hypothesis that infection with H. somnus on day 6 of experimental BRSV infection would result in disease enhancement and potentially an altered immune response when compared with single infection. Three groups of calves were either dually infected or singly infected with H. somnus or BRSV. Serum and bronchoalveolar lavage fluid (BALF) pathogen specific IgG1, IgG2, IgE, and IgA responses were evaluated by ELISA. TaqMan RT-PCR was used to examine cytokine gene expression by PBMC and BAL cells. Clinical signs were evaluated for 28 days after BRSV infection, followed by necropsy and histological examination of the lungs. In dually infected calves, disease was significantly more severe, H. somnus was isolated from the lungs at necropsy, and high IgE and IgG responses were detected to H. somnus antigens. Cytokine profiles on day 27 were elevated in dually infected calves, but did not reflect a skewed profile. These results contrasted with singly infected calves that were essentially normal by day 10 of infection and lacked both lung pathology and the presence of H. somnus in the lung at necropsy. The increase in IgE antibodies specific for antigens of H. somnus presents a possible mechanism for pathogenesis of the disease enhancement.

Introduction

Viral respiratory infections are frequently followed by bacterial infections, because virus infection results in interruption of the integrity of the epithelial barrier, decreased mucociliary activity, and increased viscosity of secretions. These changes favor bacterial persistence in the respiratory tract. For example, respiratory syncytial virus infection in humans and cattle is often followed by infection with one or more bacterial agents. In humans, Haemophilus influenzae has been associated with post-RSV infection. Like H. influenzae, Haemophilus somnus (proposed new name Histophilus somni, Angen et al., 2003) causes respiratory disease alone (Gogolewski et al., 1987) or subsequent to bovine RSV infection in cattle.

Respiratory syncytial virus (RSV) in human infants and children has been associated with development of IgE antibodies to viral proteins (Welliver et al., 1981). In laboratory animal models RSV has also been shown to enhance an IgE response to other antigens inhaled during infection (Dakhama and Bramley, 1999, Barends et al., 2002). Similarly calves infected with bovine RSV can develop an IgE response to viral proteins, as well as to other antigens encountered during infection (Stewart and Gershwin, 1989a, Stewart and Gershwin, 1989b). RSV seems to be unique among viruses in its ability to foster a cytokine environment that is supportive of an IgE response. More characteristically, virus infections promote T helper type 1 (Th1) responses for the most effective anti-viral defense. The cytokine interferon-γ (IFN-γ), a product of the T helper 1 response, exerts its anti-viral effect by stimulating NK cells and cytotoxic CD8+ T cells (Aberle et al., 1999). These responses are important for resolution of RSV infection. However, several studies have associated decreased Th1 cytokines and/or increased Th2 cytokines with severe RSV in infants. Using knockout mice, Barends et al. (2002) showed that the lack of interferon-γ signaling allowed increased Th2 cytokine production with resultant pulmonary eosinophilia and increased lung pathology in RSV infection (Welliver et al., 1981). Our group has found that anti-viral IgE levels correlate with disease severity in calves, in the presence of both Th1 and Th2 cytokine production (Gershwin et al., 2000).

IgE responses to bacterial pathogens are not common, but have been reported. In the absence of RSV infection, IgE production against H. influenzae and Streptococcus pneumoniae has been described in human cases of chronic bronchitits. Histamine release from sensitized basophils was a suggested cause of symptoms (Kjaergard et al., 1996). Others have shown similar associations of serum IgE to H. influenza, S. pneumoniae, and Staphylococcus aureus in chronic bronchitis and other forms of allergic lung disease (Pauwels et al., 1980, Tee and Pepys, 1982). Bovine IgE antibodies to Mannheimia haemolytica (formerly Pasteurella haemolytica) were induced by vaccination (Conlon and Shewen, 1993), or infection (Gershwin and Friebertshauser, 1987, McBride et al., 1992). IgE antibodies against H. somnus were detected in cattle vaccinated with H. somnus bacterin (Ruby et al., 2000).

The hypothesis in the present study was that since both bovine respiratory syncytial virus (BRSV) and H. somnus induce IgE antibodies, infection with H. somnus during an acute BRSV infection would further modulate the immune response against H. somnus towards IgE production, with consequent exacerbation of disease.