Molecular Drivers of Lymphocyte Organization in Vertebrate Mucosal Surfaces: Revisiting the TNF Superfamily Hypothesis.

The adaptive immune system of all jawed vertebrates relies on the presence of B and T cell lymphocytes that aggregate in specific body sites to form primary and secondary lymphoid structures. Secondary lymphoid organs include organized MALT (-MALT) such as the tonsils and Peyer patches. -MALT became progressively organized during vertebrate evolution, and the TNF superfamily of genes has been identified as essential for the formation and maintenance of -MALT and other secondary and tertiary lymphoid structures in mammals. Yet, the molecular drivers of -MALT structures found in ectotherms and birds remain essentially unknown. In this study, we provide evidence that TNFSFs, such as lymphotoxins, are likely not a universal mechanism to maintain -MALT structures in adulthood of teleost fish, sarcopterygian fish, or birds. Although a role for TNFSF2 (TNF-α) cannot be ruled out, transcriptomics suggest that maintenance of -MALT in nonmammalian vertebrates relies on expression of diverse genes with shared biological functions in neuronal signaling. Importantly, we identify that expression of many genes with olfactory function is a unique feature of mammalian Peyer patches but not the -MALT of birds or ectotherms. These results provide a new view of -MALT evolution in vertebrates and indicate that different genes with shared biological functions may have driven the formation of these lymphoid structures by a process of convergent evolution.