Pigs are natural hosts for swine influenza A virus (FLUAVsw) with the potential for transmitting reassorted strains to humans. Alongside neutralizing antibodies, T-cell immunity may be crucial to control FLUAVsw infections but data on this are fragmentary. Consequently, we performed a detailed characterization of the systemic and local T-cell response to influenza infection in pigs.
A total of 31 pigs were intratracheally infected with a FLUAVsw H1N2 strain and euthanized on days 2, 4, 6, 9, 12, 15 and 44 post infection (pi). 31 pigs served as controls and received PBS. Upon euthanasia, mononuclear cells/lymphocytes were isolated from blood (PBMC), lung, tracheobronchial and mesenteric lymph nodes. Isolated cells were in vitro restimulated (homologous/heterologous virus) and analyzed for intracellular IFN-γ, TNF-α and IL-2 in combination with lineage markers CD4 /CD8β and the differentiation marker CD27. Additionally, freshly isolated cells were stained for ex vivo phenotype and proliferation (Ki-67). Ex vivo flow cytometry revealed an increase of proliferating (Ki-67+) CD8β+ T cells with an early effector phenotype (perforin+CD27+) in the lung at day 6 pi. First IFN-γ producing FLUAVsw-specific T cells were detected in the lung already at 4 days pi, comprising both CD4+ and CD8β+ T cells, and peaked around day 9 pi, when influenza RNA was undetectable by RT-qPCR in bronchoalveolar lavage fluid. Also at 44 days pi, the most prominent recall responses were detected in the lung. These CD4+/ CD8β+ memory T cells were partially multifunctional and showed reactivity against a heterologous H3N2 strain in vitro.
In summary, we could show that both CD4+ and CD8β+ T cells, with different cytokine profiles and reactivity against heterologous strains, accumulate and form memory at the site of infection, the lung. Thus, our data point to a role of both T-cell populations in control of the infection.