Originally identified as a GPCR upregulated on Burkitt
Originally identified as a GPCR upregulated on Burkitt lymphoma cells upon Epstein-Barr virus infection (Birkenbach et al., 1993), Ebi2 was later found to play critical roles in immunity by regulating the migration of activated N6022 to extrafollicular sites for early plasmablast responses (Gatto et al., 2009, Gatto et al., 2011, Hannedouche et al., 2011, Kelly et al., 2011, Liu et al., 2011, Pereira et al., 2009, Yi et al., 2012) and splenic DC homeostasis via enforced localization of cDC2s in the bridging channels (Gatto et al., 2013, Lu et al., 2017, Yi and Cyster, 2013). It remains unclear why disruption of the Ebi2-oxysterol axis had no effect on LN DCs. It is tempting to speculate that these differential effects may relate to varying signals brought into the LN by lymph as compared to blood flowing into the spleen. Regardless, our findings, together with those summarized above, suggest that oxysterol gradients are utilized by multiple immune subsets to optimize cellular positioning and cooperation. Our results indicate that 7a,25-OHC plays a dominant role in coordinating immunity within LNs. It is conceivable that a gradient of 7a,25-OHC emanating from a specialized stromal cell population located at the periphery of the T cell zone (Rodda et al., 2018, Yi et al., 2012) underlies the CD4+ T cell positional bias. Whether this gradient is modulated during immune responses remains unknown. It is clear, however, that adjustments in Ebi2 expression as immune cells become activated and differentiate (Li et al., 2016, Pereira et al., 2009) help drive the range of dynamic changes required for optimal communication between sparse numbers of responding cells. It was recently shown that Gpr183−/− CD4+ T cells are impaired in their ability to differentiate to a T follicular helper (Tfh) cell phenotype due to deficient interaction with IL-2-quenching cDC2s (Li et al., 2016). While we have also observed inefficient Tfh cell production in the context of Ebi2 deficiency, we suggest an alternative and/or additional explanation for this deficit. As Tfh cell fate correlates with antigen dose (DiToro et al., 2018, Tubo et al., 2013), Ebi2 may support Tfh cell differentiation by promoting CD4+ T cell engagement with the DCs displaying the greatest concentration of peptide-MHC class II. The subsequent upregulation of CD25 expression on the engaged DCs (Li et al., 2016) would, in turn, scavenge the IL-2 produced, amplifying the existing bias toward a Tfh cell fate (Ballesteros-Tato et al., 2012, DiToro et al., 2018). The nonrandom organization of CD4+ and CD8+ T cells we report is part of a larger picture that involves how LN microanatomy supports optimized immune responses. In concert with previous results (reviewed by Qi et al., 2014), we place our findings in the context of a multilayered “fractal-like” model of secondary lymphoid tissue organization for efficient antigen recognition. The first organizational layer encompasses the formation of secondary lymphoid organs to co-localize antigen, antigen-presenting cells, and antigen-responsive lymphocytes in a limited tissue volume. The second layer includes the bulk organization of lymphocytes into separate B cell and T cell areas that segregates lymphocytes with opposing requirements for antigen recognition—intact antigen in the case of B cells versus processed small antigenic peptides for T cells. The third organizational level, which this study reveals, involves the distribution bias that matches the specialization in antigen presentation of distinct DC subsets to the antigen-recognition requirements of T cells. cDC2s and CD4+ T cells (that communicate via MHC class II molecules) occupy a paracortical “niche” separate from cDC1s and CD8+ T cells (that utilize MHC class I interactions). The present results also emphasize that the adaptive immune system has evolved to favor early and effective activation of CD4+ T cells in particular (Gerner et al., 2017). Such hierarchical organization seems critical for optimal immunity; if CD4+ versus CD8+ T cell distributions were reversed, CD8+ T cell responses would precede those of CD4+ T cells. Consequently, CD8+ T cells would fail to receive “help” and develop into cells incapable of undergoing secondary expansion upon antigen reencounter (Janssen et al., 2005). Similarly, a delay or quantitative defect in the degree of CD4+ T cell help delivered to B cells would weaken humoral immunity (Crotty, 2015).