Regional specification of oral mucosal immunity

Abstract

In this issue of Science Immunology, Barreto de Albuquerque et al. track immune responsiveness to the foodborne pathogen Listeria monocytogenes during oral infection. Their findings extend the notion of compartmentalized immunity within the gastrointestinal tract to the oral cavity and provide previously unkown insights into regional specialization of oral immunity.

The oral cavity is a site of first encounters. Food, airborne particles, and commensal bacteria and pathogens enter the gastrointestinal tract through the mouth. Yet, how microbes and other antigens are sensed and how local immunity is regulated is less understood at this interface compared to other barrier surfaces (1). In this regard, the immunological mechanisms dictating tolerance to commensals or innocuous antigens and reactivity to pathogenic organisms at this mucosal surface are incompletely understood. Moreover, how oral immune responses to pathogens affect distal immune reactivity remains unclear.

In this issue of Science Immunology, Barreto de Albuquerque et al. (2) track immune responsiveness to the foodborne pathogen Listeria monocytogenes (Lm) during oral infection and identify that mandibular lymph nodes (mandLNs) serve as sentinel lymphoid organs to intercept Lm, contributing to early protection against infection.

Lm is a Gram-positive bacterium, which infects macrophages and hepatocytes in its target organs, the spleen and liver, and is cleared by cytotoxic CD8⁺ T cells. Using an intraoral (i.o.) Lm infection model or by feeding mice Lm-contaminated food, the authors observed rapid microbial translocation of Lm to the regional submandibular LNs with associated LN enlargement and local expansion of immune (CD45⁺) cells. Intriguingly, they found that immune cell expansion occurred within 3 days in the mandLNs, and preceded expansion in the systemic circulation and in target organs, such as the spleen.

Using a green fluorescent protein (GFP)–expressing Lm (Lm-GFP) and intravital imaging, the authors show that after i.o. ingestion, Lm is transferred to the regional submandibular LNs, at least in part through afferent lymphatics of the oral mucosa (Fig. 1). They further observed that transfer of Lm did not depend on active dendritic cell migration to mandLNs but appeared to occur through a passive transport mechanism.

Fig. 1. Intraoral infection with Lm through contaminated food.

(A) Lm reaches the submandibular LNs partially through the regional lymphatic network. Within the first 1 to 2 days after i.o. infection, CD8⁺, antigen- specific T_EFF cells expand in the regional lymphatics (mandLNs) and disseminate into the circulation. (B) Oral-LN–primed T_EFF cells display specific patterns of dissemination and traffic to the spleen, oral mucosa, and lung, providing protection to distal organs such as the spleen. However, mandLN T_EFF cells do not display a gut-homing phenotype, suggesting regional specialization of T_EFF cells induced by i.o. infection with Lm.

The authors asked whether immune responses in the mandLNs were specific to Lm. Applying flow cytometry and in vivo imaging, they documented expansion of antigen-specific CD8⁺ T cells that clustered around CD11c⁺ antigen-presenting cells and displayed movement dynamics consistent with cognate antigen recognition (Fig. 1). Antigen- specific CD8⁺ T cells in mandLNs expressed markers of effector T (T_EFF) cells, became activated, and proliferated before T_EFF cells were detected in the spleen, mesenteric lymph nodes (MLNs), and Peyer’s patches (PPs). In fact, mandLN-primed CD8⁺ T_EFF cells constituted most of the circulating T_EFF cells at early time points after oral exposure and disseminated to the spleen, lung, and oral mucosa (Fig. 1). However, Unlike CD8⁺ T_Eff Cells Primed In Mlns After Intragastric Lm Gavage, Mandln CD8⁺ T_EFF Cells Did Not Express Gut-Homing Genes And Did Not Traffic To The Small Intestine. Consistent With Homing Patterns, Primed Mandln CD8⁺ T_EFF cells contributed to the control of Lm infection in the spleen and lung but not in MLNs or PPs, supporting the notion that the mucosal immune system is compartmentalized.

These findings suggest that induction of T_EFF cell responses to particular pathogens can be primed in the oral draining LNs and thereafter contribute to systemic immunity. In addition, the study reveals a regional specialization of T_EFF cells induced in the mandLNs in response to Lm, with oral- induced T_EFF cells displaying local imprinting that affects their dissemination pattern and, in turn, their ability to confer protection to select organs.

The fact that effector responses to oral-encountered antigens does occur and can contribute to systemic immune protection has been long recognized and used both for the induction of tolerogenic responses in the context of desensitization to allergens and for the induction of protective immunity in the setting of sublingual vaccination. The sublingual mucosa in the oral cavity allows for rapid transport of protein antigens that can cross the oral mucosa and circulate to regional LNs, to quickly elicit adaptive responses (1 to 3 days). Sublingual/oral mucosal delivery has been used extensively in preclinical models and has moved into clinical trials for vaccination against both viruses (influenza and human papillomavirus) and bacteria (enterotoxigenic Escherichia coli and tuberculosis) (3). More recently, induction of regional responses in the submandibular LNs have been demonstrated after i.o. infection with the foodborne parasite Trypanosoma cruzi, the cause of Chagas disease (4).

The current study not only supports time-honored notions that the oral cavity is an integral inductive and effector component of the mucosal immune system (5) but also further extends previous knowledge in the field, demonstrating induction of T_EFF cell responses to a specific orally transmitted pathogen (Lm). Most importantly, this body of work reveals insights related to tissue specification of oral-induced responses. Regional immune specificity has been demonstrated in barrier tissues such as the skin (6) and gut (7) and even within anatomically distinct segments of the gastrointestinal tract such as the small intestine and colon (8). This work further extends the concept that within the “common” and interconnected mucosal system of the gastrointestinal tract, there is unique regional immune specificity that extends to the oral environment.

The authors attributed tissue imprinting of oral T_EFF cell responses, at least in part, to the divergent functionality of stromal cells and antigen-presenting cells between the small intestine and oral-draining LNs. They observed that mandLN stromal and dendritic cells expressed lower levels of retinoic acid–producing enzymes that are required for the induction of the gut-seeking phenotype in T_EFF cells (9), and consequently, effector CD8⁺ T cells generated after i.o. infection lacked gut-homing receptors. How tissue resident cells influence the development and function of T_EFF cells in the oral draining LNs is not well understood, and the current study begins to interrogate the contribution of stromal cell populations to the unique immunological microenvironment of regional LNs in the gastrointestinal tract.

Last, although the present study is focused on a specific infection in an experimental mouse model, it addresses broader questions of interest with high relevance to human health. Understanding regional specialization of immune responses within the compartments of the gastrointestinal tract can become tremendously valuable for the design of vaccines and therapeutics delivered at particular regions of the mucosal system. Furthermore, expanding our understanding of how regional responses to orally encountered microbes affect systemic immunity in health and disease may aid in our treatment of both oral diseases and related comorbidities. Oral commensals and pathobionts are associated both with not only oral disease but also systemic illness, from cancer to autoimmunity and cardiovascular disease. In fact, systemic host responses to oral pathobionts have been epidemiologically linked to oral and systemic comorbidities. Specifically, oral microbes associated with the prevalent oral disease periodontitis have been extensively documented to elicit systemic inflammatory response that correlates with distal comorbidities (10). Thus, expanding our understanding of the induction and specification of oral-induced immune responses may open the door to therapeutic intervention toward either expansion or restraint of these responses, with potential application for the treatment of both oral and systemic diseases.