Putative Immunological Functions of Inducible Skin-Associated Lymphoid Tissue in the Context of Mucosa-Associated Lymphoid Tissue

Abstract

Acquired immunity is orchestrated in various lymphoid organs, including bone marrow, thymus, spleen, and lymph nodes in humans. However, mucosa-associated lymphoid tissue (MALT) is evolutionally known to be emerged in the oldest vertebrates as an immunological tissue for acquired immunity, much earlier than the advent of lymph nodes which appeared in endotherms. Furthermore, the lymphocytes which developed in MALT are known to circulate within the limited anatomical areas. Thus, MALT is comprehended as not the structure but the immune network dedicated to local immunity. As for the skin, skin-associated lymphoid tissue (SALT) was previously postulated; however, its existence has not been proven. Our group recently showed that aggregations of dendritic cells, M2 macrophages, and high endothelial venules (HEVs) are essential components to activate effector T cells in the murine contact hypersensitivity model and termed it as inducible SALT (iSALT) since it was a transient entity that serves for acquired immunity of the skin. Furthermore, in various human skin diseases, we reported that the ectopic formation of lymphoid follicles that immunohistochemically analogous to MALT and regarded them as human counterparts of iSALT. These data raised the possibility that SALT can exist as an inducible form, namely iSALT, which shares the biological significance of MALT. In this article, we revisit the evolution of immunological organs and the related components among vertebrates to discuss the conserved functions of MALT. Furthermore, we also discuss the putative characteristics and functions of iSALT in the context of the MALT concept.

Vav1 Promotes B-Cell Lymphoma Development

“…One possible example is that of Mucosa-associated lymphoid tissue (MALT) lymphomas that originate in sites of chronic epithelial inflammation in various sites of the body, where it plays a role in regulating mucosal immunity [41]. MALT lymphomas are present in the gastrointestinal tract [42], nasopharynx [43], thyroid [44], breast [45], lung [46], salivary glands [47], eye [48], and skin [49] and exhibit characteristics with B cells located in the marginal zone of lymph node follicles. MALT lymphomas originate in sites of chronic epithelial inflammation.…”

Colitis and Colorectal Carcinogenesis: The Focus on Isolated Lymphoid Follicles

“…Based on anatomic location, MALT is classified into several subgroups, such as bronchus-associated, conjunctiva-associated, gut-associated, larynx-associated, and nose-associated lymphoid tissues [ 2 ]. Skin-associated lymphoid tissue [ 3 , 4 ] and vascular-associated lymphoid tissue [ 5 , 6 ] are other distinct types of lymphoid tissue ( Figure 1 ).…”

Multidistrict Host–Pathogen Interaction during COVID-19 and the Development Post-Infection Chronic Inflammation

“…Mucosal surfaces are characterized by the presence of lymphoid cells that are part of a MALT, which, according to the different areas, becomes gut-associated lymphoid tissues (GALTs) in the intestine [ 84 ], bronchus-associated lymphoid tissue (BALT) in the lungs [ 85 ], skin-associated lymphoid tissue (SALT) in the skin [ 86 ], nasal-associated lymphoid tissues (NALT) in the nose [ 87 ], larynx-associated lymphoid tissue (LALT) in the larynx, and conjunctiva-associated lymphoid tissue (CALT) in the conjunctiva [ 88 ]. The role of the mucosal immune system is to protect the body’s internal surfaces while keeping the physiological functions of the relevant organs intact and efficient, such as food absorption, gas exchange, and sensory and reproductive activities [ 89 ].…”