Nasal immunity: from understanding CD8+ T cell accumulation following vaccination to exploring the role of nociceptors in nasal infection

Abstract

Many respiratory pathogens first take their foothold in the nasal mucosa (NM), from which they can replicate, disseminate, and cause life-threatening complications. Having a strong line of defense at this primary site of infection can substantially help curb disease severity. The barrier function of the nasal mucosa rests on several actors from innate and adaptive immune cells to structural cells and secreted factors. In an effort to better understand nasal immunity and develop strategies to enhance immune defense at this site, we first explore how long term CD8+ T cell mediated immunity is established in the nose and in a second chapter ask the question of whether sensory neurons, particularly nociceptors, participate in nasal immune defense. Tissue resident adaptive immune cells, particularly CD8+ Tissue resident memory (TRM) T cells, have been shown to provide rapid pathogen control upon infection. However, current clinical peripheral vaccination strategies do not elicit strong CD8+ TRM responses in barrier tissues. Here, using model antigen ovalbumin with adjuvant poly(I:C), we describe a peripheral vaccination strategy coupled to a noninvasive intranasal engagement step that allows the formation of CD8+ TRMs capable of significantly reducing viral burden following Influenza A virus (IAV) infection. We dissect the steps leading to robust effector CD8+ T cell accumulation starting with the recruitment of a small population of antigen-specific effector CD8+ T cells to the non-inflamed nasal mucosa, which we call pioneer cells. We show that following antigen delivery pioneer cells produce IFNγ leading to CXCL9 and CXCL10 production by neighboring cells, which allows further recruitment of effector CD8+ T cells. We also show that blocking α4 integrin prevented recruitment of pioneer cells to the nasal mucosa. We further investigate the molecular identity of peripheral cells generated following vaccination using single cell RNA sequencing and identify a putative pioneer cell population characterized by high Itgb4, Itgb1, as well as Runx1 expression and transcriptional activity. In the next chapter, we examine the role of nociceptors, which have increasingly been shown to modulate host defense in other barrier tissues, in nasal host defense. We first characterize sensory neuron populations in the nasal mucosa, showing dense nociceptive innervation in the respiratory tissue, as well as around the Nasal-Associated Lymphoid Tissue (NALT) and sparser nociceptive innervation in the olfactory tissue. We address the role of nociceptors in shaping nasal immunity by using genetic, systemic or local depletion of nociceptors and follow up with nasal infection models using IAV or Streptococcus pneumoniae. Following nasal influenza infection, there was no difference in viral titers between wild-type and nociceptor-ablated mice but there were trends towards more neutrophils and macrophages in nociceptor-ablated mice. However, following S. pneumoniae infection, we obtained mixed results with several independent experiments showing significantly lower bacterial burden in nociceptor depleted animals (locally and systemically), reminiscent of studies in the lungs and meninges, while more recent experiments show no difference in titers despite successful nociceptor ablation. Overall, this investigation into two very different aspects of nasal immunity revealed in one part that boosting existing known effectors of host defense namely CD8+ TRMs can be crucial against viral pathogens while in a second part showed that although the nasal tissue is densely innervated by somatosensory neurons, the role of nociceptive fibers in nasal host defense remains to be ascertained and perhaps needs to be evaluated while accounting for variables such as their interactions with olfactory neurons, or the microbial communities of the nasal cavity.