Lynn M. Corcoran

Regulatory T-cell suppressor program co-opts transcription factor IRF4 to control TH2 responses

In the course of infection or autoimmunity, particular transcription factors orchestrate the differentiation of TH1, TH2 or TH17 effector cells, the responses of which are limited by a distinct lineage of suppressive regulatory T cells (Treg). Treg cell differentiation and function are guided by the transcription factor Foxp3, and their deficiency due to mutations in Foxp3 results in aggressive fatal autoimmune disease associated with sharply augmented TH1 and TH2 cytokine production. Recent studies suggested that Foxp3 regulates the bulk of the Foxp3-dependent transcriptional program indirectly through a set of transcriptional regulators serving as direct Foxp3 targets. Here we show that in mouse Treg cells, high amounts of interferon regulatory factor-4 (IRF4), a transcription factor essential for TH2 effector cell differentiation, is dependent on Foxp3 expression. We proposed that IRF4 expression endows Treg cells with the ability to suppress TH2 responses. Indeed, ablation of a conditional Irf4 allele in Treg cells resulted in selective dysregulation of TH2 responses, IL4-dependent immunoglobulin isotype production, and tissue lesions with pronounced plasma cell infiltration, in contrast to the mononuclear-cell-dominated pathology typical of mice lacking Treg cells. Our results indicate that Treg cells use components of the transcriptional machinery, promoting a particular type of effector CD4+ T cell differentiation, to efficiently restrain the corresponding type of the immune response.

Bcl-2 can rescue T lymphocyte development in interleukin-7 receptor-deficient mice but not in mutant rag-1-/- mice.

Signals from cytokine and antigen receptors play crucial roles during lymphocyte development. Mice lacking interleukin-7 receptor are lymphopenic, due to a defect in cell expansion at an early stage of differentiation, and the few mature T cells that develop in IL-7R-/- animals are functionally impaired. Both defects were rescued completely by overexpression of the anti-apoptosis protein Bcl-2. T cell progenitors lacking antigen receptor molecules are also blocked in differentiation and die, presumably because they fail to receive a positive signal via their pre-T cell receptor. Surprisingly, Bcl-2 did not promote survival or differentiation of T cells in rag-1-/- mice. These results provide evidence that blocking apoptosis is the essential function of IL-7R during differentiation and activation of T lymphocytes and that pre-TCR signaling blocks a pathway to apoptosis that is insensitive to Bcl-2.

IL-21 regulates germinal center B cell differentiation and proliferation through a B cell–intrinsic mechanism

Germinal centers (GCs) are sites of B cell proliferation, somatic hypermutation, and selection of variants with improved affinity for antigen. Long-lived memory B cells and plasma cells are also generated in GCs, although how B cell differentiation in GCs is regulated is unclear. IL-21, secreted by T follicular helper cells, is important for adaptive immune responses, although there are conflicting reports on its target cells and mode of action in vivo. We show that the absence of IL-21 signaling profoundly affects the B cell response to protein antigen, reducing splenic and bone marrow plasma cell formation and GC persistence and function, influencing their proliferation, transition into memory B cells, and affinity maturation. Using bone marrow chimeras, we show that these activities are primarily a result of CD3-expressing cells producing IL-21 that acts directly on B cells. Molecularly, IL-21 maintains expression of Bcl-6 in GC B cells. The absence of IL-21 or IL-21 receptor does not abrogate the appearance of T cells in GCs or the appearance of CD4 T cells with a follicular helper phenotype. IL-21 thus controls fate choices of GC B cells directly.

Cytokine-Mediated Regulation of Human B Cell Differentiation into Ig-Secreting Cells: Predominant Role of IL-21 Produced by CXCR5+ T Follicular Helper Cells

Differentiation of B cells into Ig-secreting cells (ISC) is critical for the generation of protective humoral immune responses. Because of the important role played by secreted Ig in host protection against infection, it is necessary to identify molecules that control B cell differentiation. Recently, IL-21 was reported to generate ISC from activated human B cells. In this study, we examined the effects of IL-21 on the differentiation of all human mature B cell subsets—neonatal, transitional, naive, germinal center, IgM-memory, and isotype-switched memory cells—into ISC and compared its efficacy to that of IL-10, a well-known mediator of human B cell differentiation. IL-21 rapidly induced the generation of ISC and the secretion of vast quantities IgM, IgG and IgA from all of these B cell subsets. Its effect exceeded that of IL-10 by up to 100-fold, highlighting the potency of IL-21 as a B cell differentiation factor. Strikingly, IL-4 suppressed the stimulatory effects of IL-21 on naive B cells by reducing the expression of B-lymphocyte induced maturation protein-1 (Blimp-1). In contrast, memory B cells were resistant to the inhibitory effects of IL-4. Finally, the ability of human tonsillar CD4+CXCR5+CCR7− T follicular helper (TFH) cells, known to be a rich source of IL-21, to induce the differentiation of autologous B cells into ISC was mediated by the production of IL-21. These findings suggest that IL-21 produced by TFH cells during the primary as well as the subsequent responses to T cell-dependent Ag makes a major contribution to eliciting and maintaining long-lived humoral immunity.

Plasma Cell Ontogeny Defined by Quantitative Changes in Blimp-1 Expression

Plasma cells comprise a population of terminally differentiated B cells that are dependent on the transcriptional regulator B lymphocyte–induced maturation protein 1 (Blimp-1) for their development. We have introduced a gfp reporter into the Blimp-1 locus and shown that heterozygous mice express the green fluorescent protein in all antibody-secreting cells (ASCs) in vivo and in vitro. In vitro, these cells display considerable heterogeneity in surface phenotype, immunoglobulin secretion rate, and Blimp-1 expression levels. Importantly, analysis of in vivo ASCs induced by immunization reveals a developmental pathway in which increasing levels of Blimp-1 expression define developmental stages of plasma cell differentiation that have many phenotypic and molecular correlates. Thus, maturation from transient plasmablast to long-lived ASCs in bone marrow is predicated on quantitative increases in Blimp-1 expression.

The generation of antibody-secreting plasma cells

The regulation of antibody production is linked to the generation and maintenance of plasmablasts and plasma cells from their B cell precursors. Plasmablasts are the rapidly produced and short-lived effector cells of the early antibody response, whereas plasma cells are the long-lived mediators of lasting humoral immunity. An extraordinary number of control mechanisms, at both the cellular and molecular levels, underlie the regulation of this essential arm of the immune response. Despite this complexity, the terminal differentiation of B cells can be described as a simple probabilistic process that is governed by a central gene-regulatory network and modified by environmental stimuli.

The Lymphoid Past of Mouse Plasmacytoid Cells and Thymic Dendritic Cells

There has been controversy over the possible lymphoid origin of certain dendritic cell (DC) subtypes. To resolve this issue, DC and plasmacytoid pre-DC isolated from normal mouse tissues were analyzed for transient (mRNA) and permanent (DNA rearrangement) markers of early stages of lymphoid development. About 27% of the DNA of CD8+ DC from thymus, and 22–35% of the DNA of plasmacytoid pre-DC from spleen and thymus, was found to contain IgH gene D-J rearrangements, compared with 40% for T cells. However, the DC DNA did not contain IgH gene V-D-J rearrangements nor T cell Ag receptor β gene D-J rearrangements. The same DC lineage populations containing IgH D-J rearrangements expressed mRNA for CD3 chains, and for pre-Tα. In contrast, little of the DNA of the conventional DC derived from spleen, lymph nodes, or skin, whether CD8+ or CD8−, contained IgH D-J rearrangements and splenic conventional DC expressed very little CD3ε or pre-Tα mRNA. Therefore, many plasmacytoid pre-DC and thymic CD8+ DC have shared early steps of development with the lymphoid lineages, and differ in origin from conventional peripheral DC.

Homologous Recombination within Subtelomeric Repeat Sequences Generates Chromosome Size Polymorphisms in P. falciparum

We present restriction maps for chromosomes 1 and 2 of six cloned lines of P. falciparum. These delineate the locations of eight genetic markers, including genes for five antigens. In parasites from diverse areas, chromosome structure is conserved in central regions but is polymorphic both in length and sequence near the telomeres. The telomeres and adjacent sequences comprise a conserved structure at the ends of most P. falciparum chromosomes. However, the subtelomeric zones are polymorphic and coincide with the locations of a repetitive element (rep20). Deletions of rep20 generate clones of P. falciparum that lack rep20 on one or both ends of chromosomes 1 or 2, and larger deletions remove telomere-proximal genes as well. The chromosome length polymorphisms can therefore be largely explained by recombination within these blocks of repeats, a mechanism that is also important in the generation of diversity in genes for repetitive antigens of P. falciparum.

Plasma cell development and survival.

Summary:  Plasma cells have long been recognized as the basis of humoral immunity, yet we are only now beginning to appreciate the complexities of plasma cell development and the fact that not all plasma cells are created equal. In vivo, plasma cells can arise from two developmental routes: one occurring outside the follicle and another within the germinal center. A B cell’s decision to follow one of these pathways is in part determined by the phenotypic subset to which it belongs and is also influenced by the nature of the antigen eliciting the response and the affinity of the B‐cell receptor for that antigen. Once a plasma cell has chosen one of these pathways, the outcome of differentiation is relatively hard‐wired. However, the phenotype of the plasma cells arising from these two pathways is distinct in terms of survival, location, and the quantity and quality of antibody they secrete. The extra‐follicular pathway represents a relatively unchecked route to differentiation resulting in the generation of short‐lived plasma cells that secrete low‐affinity antibody. The germinal center response, however, allows the integration of external signals to delay plasma cell differentiation, eventually generating a plasma cell that secretes high‐affinity antibody of an appropriate class, and that persists for a lifetime. The means by which these varying properties are conferred to a developing plasma cell are the subject of intense investigation.

Evidence from the generation of immunoglobulin G–secreting cells that stochastic mechanisms regulate lymphocyte differentiation

Naive B lymphocytes undergo isotype switching and develop into immunoglobulin-secreting cells to generate the appropriate class and amount of antibody necessary for effective immunity. Although this seems complex, we report here that the generation of immunoglobulin G–secreting cells from naive precursors is highly predictable. The probabilities of isotype switching and development into secreting cells change with successive cell divisions and interleave independently. Cytokines alter the probability of each differentiation event, while leaving intact their independent assortment. As a result, cellular heterogeneity arises automatically as the cells divide. Stochastic division-linked regulation of heterogeneity challenges the conventional paradigms linking distinct phenotypes to unique combinations of signals and has the potential to simplify our concept of immune complexity considerably.