Follicular helper T cells expressing Blimp1 are specialized for plasma cell differentiation
Authors: Miller Karen B. , Shuparski Andrew G. , Higgins Brett W. , Sharma Siddhartha M. , Milpied Pierre J. , McHeyzer-Williams Louise J. , McHeyzer-Williams Michael G. 
Published: April 2022

AbstractB cells differentiate into antibody-producing plasma cells (PC) and germinal center (GC) B cells under the guidance of specialized CD4+ follicular helper T (TFH) cells. Here, we demonstrate that CD4 T cells require Prdm1 expression for both early PC differentiation and post-GC PC formation. Using dual Blimp1/Foxp3 reporter mice and single cell-indexed analysis, we segregate persistent compartments and expressed transcriptional programs of Blimp1+ CXCR5+PD1hi TFH (referred to here as PC-TFH) from canonical Blimp1-Bcl6+ TFH (GC-TFH) and Blimp1+Foxp3+ TFR immune regulators. Antigen recall expands localized PC-TFH compartments with rapidly divergent antigen-specific memory PC-TFH and GC-TFH programs. Thus, Blimp1 is a central mediator of PC-TFH function producing specialized TFH subsets that co-ordinate with GC-TFH function to establish high-affinity long-lasting protective immunity to vaccines and infection.One-Sentence SummaryBlimp1 expressing TFH cells express unique transcriptional programs to control PC formationRESEARCH ARTICLE SUMMARYIntroductionAdaptive B cell immunity rapidly emerges to form plasma cells (PC) for antibody production and non-PC that enter germinal centers (GC) to evolve higher affinity B cell receptors. Both pathways are essential to long-term high-affinity immune protection. The early PC to GC cell fate division is driven by B cell expression of mutually antagonist transcriptional repressors Blimp1 and Bcl6. This dichotomous B cell outcome is orchestrated through antigen-specific contact by follicular helper T (TFH) cells that express Bcl6 to upregulate CXCR5, localize into B cell regions and express transcriptional programs that influence B cell fate and function. It remains unclear what TFH cell mechanisms differentially impact these divergent B cell pathways.RationaleBlimp1 is found in Foxp3+ follicular regulatory T (TFR) cells known to impact GC B cell outcomes and play a role controlling antibody-mediated autoimmunity. In the context of infection, induced Blimp1 expression in CD4 T cells is expressed by conventional non-TFH effector cell compartments. Blimp1 segregates with emigrant CD4 T cells that leave the reactive lymphoid tissue to control innate immune function at the site of antigen entry. Conversely, Bcl6 is predominantly expressed in the GC regulating TFH pathway and is demonstrated to suppress Blimp1 expression. Germline ablation of Bcl6 exaggerates type 2 effector TH cell functions that promote excessive antibody production in the absence of the GC reaction. Similarly, loss of Bcl6 in CD4 T cells abrogates GC formation and post-GC PC responses, however multiple recent reports indicate continued support for antibody production without a Bcl6+ TFH compartment. To reconcile these findings, we propose a division of TFH function with separable pathways to regulate PC and GC differentiation. We hypothesize a central role for persistent CD4 T cell expressed Blimp1 that segregates early TFH transcriptional control to create an effector cell program that selectively targets PC differentiation.ResultsDirect intracellular staining for protein, confirmed with single Blimp1 and dual (Foxp3) reporter mice, identified Blimp1 expressing CXCR5+PD1hi TFH and TFR subsets within the spleen, bone marrow and other lymphoid tissues at steady-state. Conditional deletion of Prdm1 in CD4 T cells and adoptive transfer into immunodeficient hosts with splenic B cells, truncated both early pre-GC and late post-GC formation of PC providing a causal link to both pathways of differentiation in vivo. Across steady-state splenic T cells, in vitro activated Blimp1+CD25- CD4 T cells in T-B cell co-cultures correlated with significant levels of PC induction. Integrated single cell-indexed strategies segregate the transcriptional programs of Blimp1 expressing TFH cells (referred to here as PC-TFH) from canonical GC-inducing Bcl6+ TFH cells (GC-TFH), both distinct from Blimp1+ TFR cell programs in the steady-state. Immunization and recall produce follicular localized PC-TFH with pMHCII-tetramer binding memory response TFH cells that segregate across PC-TFH and GC-TFH compartments re-iterating the dichotomous transcriptome seen at steady-state.ConclusionThis study identifies Blimp1 as a key mediator of PC-TFH cells that sub-specialize as inducers of PC differentiation and bifurcate from the Bcl6+ GC-TFH cell pathway and functions. Persistent PC-TFH compartments assort across multiple lymphoid tissues at steady-state and are distinct from Foxp3+Blimp1+ TFR immune regulators. While PC TFH cells alone are required for early and rapid antibody responses, both TFH sub-classes are essential to the generation of high-affinity long-lived and memory response PC compartments. Cellular organization and molecular components of the PC-TFH transcriptional program indicate functional sub-specialization that can be separately targeted for immunotherapeutic purposes and adjuvant design in future vaccines.Sub-specialized Blimp1+ PC-TFH cells control PC differentiationAdaptive immune protection requires balancing the evolution of BCR affinity within germinal center (GC) B cells and the differentiation of plasma cells (PC) for production of antibodies. Both functional B cell pathways require the antigen-specific induction of specialized CD4+ follicular T (TFH) cells. Within GC-inducing TFH cells, Bcl6 is required to drive the formation and function of GC B cells. Here, we segregate PC-inducing TFH cells that require Blimp1 as a key mediator of antigen-specific PC differentiation. The Blimp1+ PC-TFH transcriptional program diverges from Bcl6+ GC-TFH compartment and Blimp1+Foxp3+ follicular regulatory T (TFR) compartments. Antigen-specific PC-TFH emerge and segregate rapidly from GC-TFH after priming and recall to co-operatively induce effective long-term adaptive immunity.