Background Sublingual immunotherapy (SLIT) is safe and reduces both symptoms and medication requirements in patients with type I respiratory allergies. Nonetheless, immune mechanisms underlying SLIT need to be further documented.
Objective A detailed characterization of the lingual immune system was undertaken in mice, to investigate the presence of tolerogenic and pro-inflammatory mechanisms.
Methods Immune cells were characterized in lingual tissues from BALB/c mice using immunohistology and flow cytometry. Resident CD4+ T cells were sorted and toll-like receptor (TLR) expression profiles as well as functional characterization were assessed by RT-PCR, T cell suppressive assays and cytokine gene expression, respectively.
Results Eosinophils and mast cells were only detected in submucosal tissues. No NK, NK-T, γ/δ, CD8+ T cells, nor B-lymphocytes were detected. Potential antigen presenting cells include various subsets of dendritic cells (CD207+ Langerhans cells, CD11b+CD11c+ myeloid cells and 120G8+ plasmacytoid DCs) together with F4/80+ macrophages. Noteworthy, both CD103− and CD103+ CD4+ T cells expressing TLR2 and TLR4 receptors are present along the lamina propria, in vicinity of myeloid CD11b+CD11c± dendritic cells. Such resident lingual CD4+ T lymphocytes comprise both suppressive T cells as well as cells with memory/effector functions (i.e. expressing IFNγ, IL4, IL10 and IL17 genes following stimulation), irrespective of the presence of the mucosal addressing marker CD103.
Conclusion The sublingual route is pertinent to induce antigen-specific tolerance, due to (i) limited numbers of pro-inflammatory cells, rather located in submucosal tissues, (ii) co-localization of APCs and resident CD4+ T cells with regulatory functions. Since the oral immune system can also elicit pro-inflammatory effector responses, the cytokine milieu in which allergens are presented by sublingual APCs needs to be controlled during immunotherapy (e.g. with adjuvants) in order to favour tolerance over inflammation.