Germinal centers (GCs) are the site of antibody diversification and affinity maturation, and as such are vitally important for humoral immunity. The study of GC biology has undergone a renaissance in the past 10 years, with a succession of findings that have transformed our understanding of the cellular dynamics of affinity maturation. In this review, we discuss recent developments in the field, with special emphasis on how GC cellular and clonal dynamics shape antibody affinity and diversity during the immune response.

Antibodies somatically mutate to attain high affinity in germinal centers (GCs). There, competition between B cell clones and among somatic mutants of each clone drives an increase in average affinity across the population. The extent to which higher-affinity cells eliminating competitors restricts clonal diversity is unknown. By combining multiphoton microscopy and sequencing, we show that tens to hundreds of distinct B cell clones seed each GC, and that GCs lose clonal diversity at widely disparate rates. Furthermore, efficient affinity maturation can occur in the absence of homogenizing selection, ensuring that many clones can mature in parallel within the same GC. Our findings have implications for development of vaccines in which antibodies with non-immunodominant specificities must be elicited, as is the case for HIV-1 and influenza.

Celiac disease (CD) is an immune mediated disorder in which mucosal autoantibodies to the enzyme transglutaminase 2 (TG2)1 are generated in response to the exogenous antigen gluten2 in individuals who are HLA-DQ2 or HLA-DQ83. We assessed in a comprehensive and non-biased manner the IgA anti-TG2 response by expression cloning of the antibody repertoire on ex vivo isolated intestinal antibody-secreting cells (ASCs). We found that TG2-specific plasma cells are hugely expanded in patients with active CD, representing on average 10% of ASCs within the duodenal mucosa. Surprisingly, anti-TG2 antibodies were of high affinity and yet showed little adaptation by somatic mutations. Unlike infection-induced peripheral blood plasmablasts4, the TG2-specific ASCs had neither recently proliferated nor were they short-lived ex vivo. Altogether these observations demonstrate that there is a germline repertoire with high affinity for TG2 that may favour massive generation of autoreactive B cells. Anti-TG2 antibodies did not block enzymatic activity and served as substrates for TG2-mediated crosslinking when expressed as IgD or IgM, but not as IgA1 or IgG1. This could result in preferential recruitment of plasma cells from naïve IgD/IgM-expressing B cells, thus possibly explaining why the anti-TG2 response bears signs of a primary immune response despite the disease chronicity.

Highlights d Germline transcripts peak prior to GC formation and rapidly decline in GCs d IgM-dominated clones are found in late GCs, arguing against ongoing Ig switching d CSR largely ceases upon the onset of somatic hypermutation d CSR decline due to low GLT and APE1 expression is possibly orchestrated by BCL6

SUMMARY During antibody affinity maturation, germinal center (GC) B cells cycle between affinity-driven selection in the light zone (LZ) and proliferation and somatic hypermutation in the dark zone (DZ). Although selection of GC B cells is triggered by antigen-dependent signals delivered in the LZ, DZ proliferation occurs in the absence of such signals. We show that positive selection triggered by T cell help activates the mechanistic target of rapamycin complex 1 (mTORC1), which promotes the anabolic program that supports DZ proliferation. Blocking mTORC1 prior to growth prevented clonal expansion, whereas blockade after cells reached peak size had little to no effect. Conversely, constitutively active mTORC1 led to DZ enrichment but loss of competitiveness and impaired affinity maturation. Thus, mTORC1 activation is required for fueling B cells prior to DZ proliferation rather than for allowing cell cycle progression itself, and must be regulated dynamically during cyclic re-entry to ensure efficient affinity-based selection.

Highlights d Memory B cell reentry into germinal centers is rare under typical boost regimens d Most (>90%) B cells in secondary GCs have no prior GC experience d A clonality bottleneck restricts the diversity of recall antibody-producing cells d Most primary diversity is found in an MBC compartment not accessed by boosting

The intestinal immune system has the challenging task of tolerating foreign nutrients and the commensal microbiome, while excluding or eliminating ingested pathogens. Failure in such balance leads to severe diseases such as inflammatory bowel diseases (IBD), food allergies or invasive gastrointestinal infections 1 . Multiple immune mechanisms are therefore in place to maintain tissue integrity, including balanced generation of effector T (T H ) cells and FOXP3 + regulatory T (pTreg) cells, which mediate resistance to pathogens and regulate excessive immune activation, respectively 1 – 4 . The gut–draining lymph nodes (gLNs) are critical sites for orchestrating adaptive immunity to luminal perturbations 5 – 7 . However, how they manage to simultaneously support tolerogenic and inflammatory reactions is incompletely understood. Here we report that gLNs are immunologically unique according to the functional gut segment they drain. Stromal and dendritic cell gene signatures as well as T cell polarization against the same luminal antigen differed between gLNs, the proximal small intestine–draining gLNs preferentially giving rise to tolerogenic and the distal gLNs to pro-inflammatory T cell responses. This segregation permitted targeting distal gLNs for vaccination and maintenance of duodenal pTreg cell induction during colonic infection. Conversely, the compartmentalized dichotomy was perturbed by surgical removal of select distal gLNs and duodenal infection, impacting both lymphoid organ and tissue immune responses. Our findings reveal that the conflict between tolerogenic and inflammatory intestinal responses is in part resolved by discrete gLN drainage, and encourage gut segment-specific antigen targeting for therapeutic immune modulation.

Systemic lupus erythematosus (SLE) is an incurable autoimmune disease characterized by autoantibody deposition in tissues such as kidney, skin and lungs. Notably, up to 75% of patients with SLE experience neuropsychiatric symptoms that range from anxiety, depression and cognitive impairment to seizures and, in rare cases, psychosis-collectively this is referred to as central nervous system (CNS) lupus. In some cases, certain autoantibodies, such as anti-NMDAR or anti-phospholipid antibodies, promote CNS lupus. However, in most patients, the mechanisms that underlie these symptoms are unknown. CNS lupus typically presents at lupus diagnosis or within the first year, suggesting that early factors contributing to peripheral autoimmunity may promote CNS lupus symptoms. Here we report behavioural phenotypes and synapse loss in lupus-prone mice that are prevented by blocking type I interferon (IFN) signalling. Furthermore, we show that type I IFN stimulates microglia to become reactive and engulf neuronal and synaptic material in lupus-prone mice. These findings and our observation of increased type I IFN signalling in post-mortem hippocampal brain sections from patients with SLE may instruct the evaluation of ongoing clinical trials of anifrolumab, a type I IFN-receptor antagonist. Moreover, identification of IFN-driven microglia-dependent synapse loss, along with microglia transcriptome data, connects CNS lupus with other CNS diseases and provides an explanation for the neurological symptoms observed in some patients with SLE.