Early Immune Response and Cytokine Induction of PBMC‐derived Bovine Macrophages to Infection with Rift Valley Fever Virus in the Presence and Absence of Culex tarsalis Mosquito Saliva

Abstract

Rift Valley fever virus (RVFV) is a zoonotic arbovirus in the family Phenuiviridae. In Africa, outbreaks of Rift Valley fever (RVF) usually occur following periods of heavy rainfall that prompt population surges in mosquito vectors. RVF causes significant mortality in ruminants, especially in ovine fetuses and neonates (up to 100%), while human infections cause mild febrile illness to hemorrhagic fever and death. Macrophages play a critical role as phagocytes in the innate immune system and produce cytokines that stimulate the adaptive immune system. As in other arboviral diseases, macrophages and dendritic cells are thought to be early infection targets of RVFV, enabling the virus to suppress the immune response. To further investigate their role in RVF early immune response and cytokine induction, bovine peripheral blood mononuclear cell-derived macrophages were infected with RVFV MP-12 (an attenuated vaccine strain) in the presence and absence of Culex tarsalis saliva. We hypothesized that in macrophages treated with Culex saliva and RVFV will have increased transcription of Th2-associated cytokines compared to those infected with virus only due to immunomodulatory proteins present in mosquito saliva. The macrophage lineage of the cells and RVFV infection were confirmed by dual label immunofluorescence for IBA-1 (macrophage marker) and RVFV nucleoprotein. Total RNA was extracted from 3 biological replicates of macrophages treated respectively with saliva only, MP-12 only, MP-12 and saliva at 0, 8 and 24 hours post-infection (hpi) as well as untreated controls. RNA was reverse-transcribed and the resulting cDNA analyzed by qPCR of four housekeeping genes (ActinB, H3F3, PPIA, YWHAZ), 6 early response genes (TLR3, TLR7, TLR8, RIG1, MDA5, IFNa) and 9 cytokine genes (IL4, IL6, IL10, IL12b, IL18, IFNg, TNFa, IL1a, IL1b). Relative gene expression (RGE) was determined by the method described by Vandesompele which uses multiple reference genes for normalization and does not assume equal PCR efficiencies for the reference genes and genes of interest. RGEs for each target were log-transformed and analyzed with two-way ANOVAs and Tukey's post-hoc test with p-value correction for multiple tests to evaluate statistical differences between treatments and time points. There was no statistical increase in transcription of Th2-associated cytokines when bovine macrophages were infected with MP-12 in the presence of saliva at any of the time points when compared to virus only. In contrast, significant differences in gene expression were present when infected samples were compared to uninfected controls. At 8 hpi, transcription was significantly different than controls for all virus infected samples for TLR3, RIG1, MDA and IFNa as well as IL6, IL10, IL12b, IL18, TNFa, IL1a and IL1b. This trend continued at 24 hpi with the exception of IL12b. While Culex saliva may not enhance suppression of the early immune response in MP-12 infection, it may still have a role in virulent RVFV strain infections.