b Gene Ontology enrichment analysis showing the top 10 pathways to which TIA1 and TIAL1 gene targets are associated. key mechanisms are still unknown. Here, we show that this RNA binding proteins TIA1 and TIAL1 are required for the generation of long-lasting GC responses. TIA1- and TIAL1-deficient GC B cells fail to undergo antigen-mediated positive selection, growth and differentiation into B-cell clones producing high-affinity antibodies. Mechanistically, TIA1 and TIAL1 control the transcriptional identity of dark- and light-zone GC B cells and enable timely expression of the prosurvival molecule MCL1. Thus, we demonstrate R18 here that TIA1 and TIAL1 are key players in the post-transcriptional program that selects high-affinity antigen-specific GC B cells. Keywords: Adaptive immunity, Germinal centers, Post-transcriptional gene regulation, RNA binding proteins, Cell identity, Apoptosis Subject terms: Germinal centres, Gene regulation in immune cells, Immune cell death Introduction Germinal centers (GCs) are the sites in secondary lymph organs where B cells undergo affinity maturation and differentiation into long-lasting memory B cells and high-affinity antibody-producing plasma cells. Affinity maturation of GC B cells relies on the fine coordination of antigen recognition through the B-cell receptor (BCR) with activation, proliferation, somatic hypermutation (SHM) and selection of B-cell clones [1, 2]. Gene expression programs controlling each of these processes are tightly modulated in R18 GC B cells, both at the transcriptional and post-transcriptional levels [3C5]. Gene transcription is usually closely linked R18 with the functional regulation of nascent RNA through interactions with RNA binding proteins (RBPs). RBPs control the splicing, editing, translation and decay of mRNAs involved in the establishment, growth and differentiation of GC cells [6]. Genetic ablation of key RBPs, such as HuR and PTBP1, has a direct impact on the expression of grasp transcription factors (TFs) and TF-associated gene signatures in GC B cells, highlighting the importance of post-transcriptional mechanisms in GC-mediated immune responses [7C9]. HuR, PTBP1 and IGF2BP3 directly regulate the expression of MYC and the MYC-associated gene signature that mediates antigen-dependent selection and growth of GC B cells [10, 11]. In addition, these molecules secure cell?energy fueling and DNA replication and contribute to the mitigation of extensive DNA damage introduced by activation-induced deaminase (AID) in rapidly proliferating GC B cells. More recently, a genetic screen targeting RBPs identified dozens of these proteins having a relevant role in the terminal differentiation of GC B cells into memory and plasma cells [12]. This included members of the RNA deadenylation complex and decay machinery that?are central for this terminal differentiation process, and they act coordinately in the maintenance of long-lived plasma cells R18 [13, 14]. Multiple other?RBPs are believed to contribute to launching and maintaining GC-mediated immunity, as they are as abundant as TFs. However, to date, only a handful of RBPs have been shown to be relevant for the GC reaction. Identification of the RNA targets of these key RBPs often reveals dozens of functionally related mRNAs that are commonly deregulated in knockout (KO)?GC B cells [15]. Similarly, the same mRNA molecule can be bound by several proteins. Taken together, these findings suggest that RBPs must constitute networks for post-transcriptional gene regulation in GC B cells [16]. Here, we identify TIA1 and TIA-like 1 (TIAL1) as essential RBPs for the maintenance Col4a5 of long-lasting GC responses and the production of high-affinity class-switched antibodies. TIA1 and TIAL1 were first described in cancer-infiltrating cytotoxic T cells [17C19]. However, their physiological relevance in lymphocytes remains unclear. TIA1 and TIAL1 contain three RNA recognition motifs (RRMs) that recognize U-rich elements present in the introns and 3′ untranslated region (3’UTRs) of their target mRNAs and are involved R18 mostly in the regulation of mRNA splicing and translation. The association of TIA1 and TIAL1 with their RNA targets is usually often coupled with cell detection of internal and external cues, allowing timely expression of key mRNA targets for cell growth and proliferation (e.g., and as one of the key mRNA targets of TIA1?and?TIAL1 that allows the selection and survival of antigen-specific GC B cells. Materials and methods Animal.