The Role of Neutrophil Heterogeneity in Lung Injury During Acute Viral Respiratory Infections

Abstract

Neutrophils are the most common circulating leukocyte in humans and known to be essential for host defense against bacteria and fungi. Their roles in respiratory viral infections, however, are more indeterminate. Human infections with two major classes of viral pathogens, influenza A viruses (IAV) and coronaviruses, result in a wide clinical spectrum from asymptomatic to lethal infections and a major driver of severity in both types of viral infection has been found to be excessive neutrophilic inflammatory activation. Neutrophil depletions in infected mice, however, have often proven deleterious, substantiating that they play key beneficial roles during viral infections. It is still poorly understood how pathologic neutrophil responses are delineated from beneficial ones during infection, and how their functions may vary between hosts to drive susceptibility. In this dissertation, we address these gaps in understanding using comparative studies of coronavirus and influenza A virus infection in inbred mice of different genetic backgrounds with differing susceptibilities to infection. First, we used murine hepatitis virus strain 1 (MHV-1) to model severe coronavirus infections. In A/J mice, infection with MHV-1 induces severe respiratory pathology. C57BL/6J (B6) mice, however, present almost asymptomatically to the same infections. The disparity in lung pathology was found to be dependent on differences in neutrophil phenotypes – the A/J neutrophils expressed more primary granule proteins, a particularly damaging aspect of the neutrophil armament, and exhibited greater recruitment and activation in the lungs that mediated increased lung injury. In contrast, neutrophils had limited involvement during infection of B6 mice. Neutrophil depletion was found to abrogate excess lung injury in A/J mice while having no detectable effect on lung injury in B6 mice or viral loads in either strain, implicating neutrophils as a major host-specific driver of pathology. In the next study, we used A/PR8/34 H1N1 IAV (PR8) to model severe influenza infections. Again, the A/J mice proved hyper-susceptible to infection. In contrast to MHV-1 infection, however, A/J mice had deficient early neutrophil responses compared to B6 mice. To examine the effect of the early neutrophil response, we transferred A/J and B6 neutrophils into infected A/J mice. Transfer of B6 neutrophils resulted in efficient control of virus and limited dissemination of inflammation, while transfer of A/J neutrophils failed to achieve either. These results demonstrate that early neutrophil responses are pivotal in antiviral control and in limiting excessive inflammation, and that host-specific neutrophil-intrinsic traits may mediate these protective effects. Taken together, our results point out that neutrophil responses can unequivocally benefit the host, but their contributions are heavily affected by host-specific neutrophil functional variation. The degree of interindividual functional variation in neutrophils is an underexplored host factor in susceptibility to respiratory viral infections. Future studies are also needed to further profile and distinguish positive neutrophil responses from negative ones while considering that the optimal neutrophil phenotypes will vary with the temporal and inflammatory context.