FEZF2 and AIRE1: An Evolutionary Trade-off in the Elimination of Auto-reactive T Cells in the Thymus

Abstract

Autoimmune Regulator 1 (AIRE1) and Forebrain Embryonic Zinc Finger-Like Protein 2 (FEZF2) play pivotal roles in orchestrating the expression of tissue-restricted antigens (TRA) to facilitate the elimination of autoreactive T cells. AIRE1's presence in the gonads of various vertebrates has raised questions about its potential involvement in gene expression control for germline cell selection. Nevertheless, the evolutionary history of these genes has remained enigmatic, as has the rationale behind their apparent redundancy in vertebrates. Furthermore, the origin of the elimination process itself has remained elusive. To shed light on these mysteries, we conducted a comprehensive evolutionary analysis employing a range of tools, including multiple sequence alignment, phylogenetic tree construction, ancestral sequence reconstruction, and positive selection assessment. Our investigations revealed intriguing insights. AIRE1 homologs emerged during the divergence of T cells in higher vertebrates, signifying its role in this context. Conversely, FEZF2 exhibited multiple homologs spanning invertebrates, lampreys, and higher vertebrates. Ancestral sequence reconstruction demonstrated distinct origins for AIRE1 and FEZF2, underscoring that their roles in regulating TRA have evolved through disparate pathways. Furthermore, it became evident that both FEZF2 and AIRE1 govern a diverse repertoire of genes, encompassing ancient and more recently diverged targets. Notably, FEZF2 demonstrates expression in both vertebrate and invertebrate embryos and germlines, accentuating its widespread role. Intriguingly, FEZF2 harbors motifs associated with autophagy, such as DKFPHP, SYSELWKSSL, and SYSEL, a process integral to cell selection in invertebrates. Our findings suggest that FEZF2 initially emerged to regulate self-elimination in the gonads of invertebrates. As organisms evolved toward greater complexity, AIRE1 likely emerged to complement FEZF2's role, participating in the regulation of cell selection for elimination in both gonads and the thymus. This dynamic interplay between AIRE1 and FEZF2 underscores their multifaceted contributions to TRA expression regulation across diverse evolutionary contexts.

Keywords: Evolution; Thymus; Tissue-restricted antigens.