Adaptive Cellular Immunity against African Swine Fever Virus Infections

Abstract

African swine fever virus (ASFV) remains a threat to global pig populations. Infections with ASFV lead to a hemorrhagic disease with up to 100% lethality in Eurasian domestic and wild pigs. Although myeloid cells are the main target cells for ASFV, T cell responses are impacted by the infection as well. The complex responses remain not well understood, and, consequently, there is no commercially available vaccine. Here, we review the current knowledge about the induction of antiviral T cell responses by cells of the myeloid lineage, as well as T cell responses in infected animals, recent efforts in vaccine research, and T cell epitopes present in ASFV.

Keywords: African swine fever virus; T cells; adaptive immunity; antigen-presenting cells; cytokines; porcine disease; vaccines.

Figure 1

Adaptive cellular responses during ASFV infection. T cells have been shown to be particularly important for the survival of ASF infections. Antiviral T cell responses are predominantly executed by CD8α+ T cells and include cytotoxicity as well as secretion of proinflammatory cytokine-like IFNγ. The widespread loss of perforin in cytotoxic T cells potentially indicates consumption or a switch to other cytotoxic effector molecules. During ASFV infections, CD4+ Th cells support B cell responses and essential antibody (Ab) maturation. Studies on nonconventional T cells, such as γδ T cells and invariant Natural Killer T (iNKT) cells, point to a role in antigen presentation and possibly also as mediators of cytotoxic responses. The induction of adaptive responses, facilitated by professional antigen-presenting cells (APC), is also significantly disturbed during ASFV infections. Virulent ASFV strains often impair vital cellular functions, such as cytokine secretion and phagocytosis, or induce pathological cytokine responses, thereby diminishing targeted antiviral responses. Adaptive cytokines, such as anti-inflammatory IL-10 and proinflammatory IFNγ, are often found in moribund animals during likely irreversibly derailed responses and do not correlate well with protection. While both Eurasian suids, domestic pigs and wild boar, are highly susceptible to ASFV infections, experimental studies found higher disease severity and lethality in wild boar even during moderately virulent ASFV infections. Wild boar were also found to have a significant bias towards γδ T cell responses. Where applicable, effects of moderately virulent or attenuated ASFV strains were marked with a green virus (mv), and effects of highly virulent ASFV strains were marked with a red virus (hv). Created with BioRender.com (accessed on 17 February 2022).