Heligmosomoides polygyrus-induced serum factors limit respiratory syncytial virus titres: a potential role for CCL8

Abstract

Heligmosomoides polygyrus (H. polygyrus) is a naturally occurring intestinal nematode that employs immunomodulatory mechanisms towards the establishment of long term chronic infections within mice. The robust host Type 2 immune response to H. polygyrus infection can lead to a subdual of excess inflammation caused by a Type 1 immune response. Respiratory syncytial virus (RSV) is one of the most common viral respiratory infections among infants, young children, immunocompromised and elderly individuals. Infections with RSV lead to high rates of infant hospitalisation and mortality amongst low-income countries. Previously, there were no effective antiviral treatments, or vaccines for under 75’s and those who were hospitalised received supportive care. Very recently RSV vaccination for those over 75 and passive immunisation against RSV in young infants (nirsevimab, maternal vaccination) have become available. Enhancing our understanding of the anti-viral response to RSV remains important to develop therapies that limit RSV burden and disease. H. polygyrus infection in mice has protective antiviral effects against RSV in the lung through induction of a microbiota-dependent Type I interferon response. H. polygyrus infection also leads to systemic monocytosis, contributing to elevated mononuclear phagocyte numbers in the lung, that reduces peak viral load of RSV. Despite this growing understanding of the anti-viral effects of H. polygyrus, a key question remains of how a strictly enteric infection induces these pulmonary responses.

Transfer of acellular serum from H. polygyrus infected mice (H. polygyrus serum) has been found to replicate the induction of interferons and reduction of RSV burden in recipient mice. A proteomic analysis of H. polygyrus serum revealed several elevated cytokines including Ccl8. This study aimed to investigate if H. polygyrus serum drives monocytosis and if Ccl8 chemokine is required for the antiviral effects of H. polygyrus serum to be sucsessful against RSV. Following transfer of H. polygyrus serum or control serum, flow cytometry of tail vein blood was undertaken to assess induction of monocytosis. To test the role of Ccl8 in H. polygyrus serum, antibody mediated depletion of Ccl8 was performed before transfer to naïve animals and subsequent infection with RSV 24 hours post serum transfer. Viral load within the lungs was assessed using RSV immunoplaque assays.

Mice that received H. polygyrus serum depleted of Ccl8 displayed a higher viral load 4 days post RSV infection, compared to mice that received control or non-depleted H. polygyrus serum. Furthermore, flow cytometry of blood samples taken 24 and 48 hours after serum transfer revealed that mice receiving H. polygyrus serum had lower levels of classical monocytes compared to mice that received control serum. Four days after the transfer, mice that received H. polygyrus serum also showed a lower level of classical monocytes and higher levels of non-classical monocytes compared to mice that received control serum. However, it can be concluded that monocyte percentages tend to be lower with H. polygyrus serum transfer, but these differences were not statistically significant from day 1 to day 4 post serum transfer.

Based on the findings, it can be concluded that there is a recurring theme within mice receiving H. polygyrus serum displaying lower levels of classical monocytes in the blood throughout the 6 days. Additionally, the Ccl8 cytokine appears to play a crucial role in providing anti-viral protection as mice receiving H. polygyrus serum depleted of Ccl8 displayed a higher viral load compared to those receiving control or H. polygyrus serum without Ccl8 depletion. These results highlight the potential of H. polygyrus induced serum in modulating immune responses and reducing viral load, emphasising the importance of further understanding the mechanisms involved within the process of anti-viral protection.