The induction of resting B cell differentiation does not require T cell contact.

A T cell clone as well as immediately ex vivo CD4+ lymph node T cells are shown to support the differentiation of co-cultured resting B cells in the absence of T cell-B cell contact. Antibodies specific for class II products of the major histocompatibility complex inhibit the transactivation of resting but not activated B cells. This differential inhibition pattern indicates that the responses obtained from resting B cell populations are not due to their contamination with B cell blasts. Further, supernatants prepared from an activated T cell clone induce resting B cell differentiation. Two lines of evidence suggest that the activity contained in these supernatants can be attributed to interleukin (IL)-5. Activity is neutralized by monoclonal anti-IL-5; and both recombinant and affinity-purified IL-5 induce the differentiation of immediately ex vivo resting and activated B cells with comparable efficiency. Taken together, these results demonstrate that contact with T cells does not provide prerequisite signals for the induction of resting B cell differentiation.     

Stimulation of a memory B cell response does not require primed helper T cells

The use of universally immunogenic T cell epitopes, such as those identified in tetanus toxin or malaria circumsporozoite protein, could represent a major improvement in the development of synthetic vaccines. However, one limitation of this approach is the lack of T cell cross-reactivity between the vaccine and the pathogen. To determine whether the memory B cell response elicited by immunization with a synthetic peptide containing a B cell epitope linked to a T cell epitope can be restimulated by the same B cell epitope linked to different T cell epitope(s), we used a synthetic peptide which contains non-overlapping B and T cell determinants from hepatitis B surface antigen (HBsAg) of hepatitis B virus (HBV). The results of this study clearly show that primed T cells can increase the antibody response against a B cell epitope linked to the priming T cell determinant. However, the antibody response obtained was weaker than that obtained after two injections of the peptide containing both B and T cell epitopes, showing the important role played by memory B cells in secondary antibody responses. Moreover, a strong antibody response against the B cell epitope was elicited by boosting mice with the B cell epitope linked to a heterologous carrier, thus demonstrating that a strong B cell memory response can be revealed in the absence of primed T cells. These results therefore provide new important information for the design of synthetic or recombinant vaccines.     

In vitro induction of T cell-dependent B cell differentiation in patients with common varied immunodeficiency

Patients with common varied immunodeficiency (CVI) are characterized by hypogammaglobulinemia. We investigated in vitro T cell-dependent B cell differentiation in CVI peripheral blood mononuclear cells by stimulating the T cells with an anti-CD3 (T3) monoclonal antibody. In cultures from CVI patients with no detectable circulating B cells, little immunoglobulin (Ig) was produced following anti-CD3 stimulation. In cultures from CVI patients with near-normal numbers of circulating B cells, anti-CD3 stimulation induced a normal percentage increase in Ig-secreting cells and appreciable (albeit subnormal) increases in IgG and IgM secretion. Cell-mixing experiments pointed to a quantitative, rather than qualitative, defect in B cell function in most of these CVI patients. Nevertheless, CVI T cells can induce substantial differentiation of autologous (and normal) B cells following anti-CD3 stimulation (which may mimic physiologic stimulation). This raises the possibility of correcting the hypogammaglobulinemia of CVI by in vivo or ex vivo administration of appropriate T cell stimuli.     

The molecular interactions with helper T cells which limit antigen-specific B cell differentiation

Helper T (Th) cell-dependent activation requirements for 2,4,6-trinitrophenyl (TNP)-specific resting B cells obtained from mice transgenic for Sp-6 mu, kappa genes were analyzed. Carrier-specific T cell help required linked recognition of TNP carrier and was functionally restricted by the B cell major histocompatibility complex. However, histoincompatible T cell-B cell conjugates formed by bridging surface immunoglobulin and Th cell receptor for antigen (TcR) through TNP-conjugated anti-TcR antibodies resulted in the efficient differentiation of TNP-specific B cells. Thus, Th cell-dependent cognate recognition of B cells is not obligatory. Specific conjugate formation could be obviated by using unconjugated fragments of anti-TcR antibodies. If dimeric, these fragments supported the Th cell-dependent differentiation of co-cultured histoincompatible resting B cells. Unconjugated monomeric fragments were ineffective, demonstrating the necessity for TcR cross-linking. Resting B cells from Sp-6+ mice rendered TNP-conjugated monomeric fragments of anti-TcR antibodies effectively multivalent, thereby satisfying conditions for the activation of co-cultured Th cells. The results demonstrate that Th cells do not transduce activation signals through TcR recognition of B cell membrane-associated ligand which limit the induction of B cell differentiation. Cross-linking of TcR on Th cells is required, sufficient and can be induced through interaction with the antigen-specific B cell surface.     

The role of functionally distinct helper T lymphocyte subpopulations in the induction of human B cell differentiation

Human helper T lymphocytes can be dissected into two functionally distinct subpopulations based on expression of the CD45RA (2H4) or CD45R0 (UCHL-1) surface antigens. While both subpopulations are able to induce equivalent levels of B cell activation and proliferation, only the CD4+CD45RA- subpopulation is capable of inducing B cell differentiation in pokeweed mitogen (PWM)-stimulated cultures. To define the mechanism responsible for the dichotomy between induction of proliferation and differentiation by the two CD4+ subpopulations, we examined the abilities of the purified T cell subpopulations to produce lymphokine mRNA following T cell activation. Northern analysis revealed that both subpopulations produced interleukin (IL) 2 and interferon (IFN)-gamma mRNA following PWM activation. The CD4+CD45RA- subpopulation, however, produced higher levels of IFN-gamma mRNA and the CD4+CD45RA+ cells produced higher levels of IL 2 mRNA. Neither subpopulation elaborated detectable mRNA for IL 4, IL 5 or IL 6. Of greatest significance was that the addition of recombinant or T cell-derived lymphokines could not compensate for the inability of the CD4+CD45RA+ subpopulation to induce B cell differentiation in PWM assays. Direct T-B cell contact was required for the optimal induction B cell differentiation in these assays, suggesting that CD4+CD45RA+ T cells were deficient in their ability to directly deliver the T cell-B cell signals required for B cell differentiation. These results suggest that the differential ability of the two subpopulations of CD4+ T cells to induce B cell differentiation does not result from differences in lymphokines elaborated, but may result from differences in their abilities to interact directly with B cells to initiate differentiation.     

Priming of helper T cell-dependent antibody responses by hemagglutinin-transgenic B cells

Mice expressing the hemagglutinin (HA) gene of influenza virus PR8 (H1 subtype) under the control of x light chain promoter and enhancer have been generated. They express HA in and on B cells, and are tolerant to HA. In vitro, only lipopolysaccharide (LPS) blasts but not resting B cells of transgenic mice can stimulate HA-specific helper T cells of HA-specific α/β T cell receptor (TCR)-transgenic mice. Transfer of HA-transgenic LPS blasts into syngeneic, non-transgenic recipients primes HA-specific antibody responses. Resting, small HA-transgenic B cells, which were purified by fluorescence-activated cell sorting, prime lower antibody responses. Host B cells produce the HA-specific antibody response. The donor HA-transgenic B cells need to express major histocompatibility complex (MHC) class II molecules and need to be alive to induce the antibody response in the host. Most notably, the host antibody response never produces detectable levels of IgM, but only of switched IgG isotypes. Neither resting nor activated HA-transgenic B cells induce tolerance in antibody responses. These results suggest that HA-transgenic B cells, presenting both the intact antigen on the cell surface and peptides of the antigen on MHC class II, are effective inducers of helper T cell responses, and as judged by the Ig-isotype response pattern, which is mainly IgG1, of Th2 type.     

Inhibition of T cell-dependent human B cell proliferation and B cell differentiation by polyspecific monomeric IgG.

A commercially available polyspecific, monomeric IgG preparation suitable for intravenous administration (IgSRK; Sandoglobulin) can inhibit pokeweed mitogen (PWM)-induced proliferation of peripheral blood mononuclear cells (PBMC) by a small, but statistically significant, amount compared to control cultures. Such inhibition could not be demonstrated when PBMC were stimulated with the T cell mitogen phytohaemagglutinin. Surface phenotype analysis of the PWM-stimulated cells indicated that in IgSRK-containing cultures, the proportion of B cells was decreased and the proportion of T cells was increased compared to control cultures. This alteration in T:B ratio was not due to antigenic modulation of B or T cell markers from their surfaces. In addition, IgSRK inhibited the proliferation of T cell-depleted PBMC cultures stimulated by B cell proliferation factors (BCPF) but not by fixed protein A-bearing Staphylococcus aureus strain Cowan I. The capacity to inhibit B cell proliferation was independent of and distinct from its capacity to inhibit B cell differentiation, since IgSRK inhibited the differentiation of a B cell differentiation factor (BCDF)-sensitive line by BCDF (which contains no BCPF activity). IgSRK inhibited PWM-induced generation of cytoplasmic Ig+ cells but had no effect on Ig secretion from mature Ig-secreting cells. Taken together, these findings suggest that IgSRK (which contains the IgG fraction from pooled plasma from 2,000 healthy donors) can inhibit T cell-dependent or T cell factor-dependent B cell proliferation and B cell differentiation.     

Costimulation through the inducible costimulator ligand is essential for both T helper and B cell functions in T cell-dependent B cell responses

Costimulation through the inducible costimulator (ICOS) and its ligand (ICOSL) is essential for T cell-dependent B cell responses, but the cellular and temporal dynamics underlying its in vivo effects are poorly defined. Here we have shown that Icosl(-/-) and Icos(-/-) mice had similar phenotypes and that ICOS-ICOSL costimulation modulated the early but not late phases of IgG1 affinity maturation. Exploiting the adoptive transfer of T or B cells from primed Icosl(-/-) mice, we provided genetic evidence that costimulation through ICOSL was essential for primary but not secondary helper T cell responses and for the control of both T and B cell activities, resulting in T cell-dependent IgG1 production.     

Determinants of helper T cell-dependent antibody production

Multiple factors regulate the development and maintenance of humoral immune responses. The cytokines produced by helper T cells are major determinants of the magnitude and phenotype of antibody responses to protein antigens. Th2-derived cytokines are most effective at initiating primary B cell responses, whereas Th1 cells are effective helpers for B cells exposed to multivalent antigens and for previously activated B cells. In addition, different cytokines stimulate B cell switching to particular antibody isotypes. Cytokine production can be regulated in vivo by the conditions of antigen exposure, and by the nature of antigens. Tolerogenic antigens alter the phenotypes of antibody responses by preferentially inactivating selected subsets of helper T cells. Morphologic techniques provide potentially valuable approaches for studying T/B cell interactions and the nature and consequences of local cytokine production in situ, in lymphoid tissues.     

辅助T细胞分化的表观遗传学调控

在抗原的刺激下,CD4^＋辅助性T细胞（Th细胞）可分化为不同的细胞亚群以对抗不同的外界环境。Th细胞的分化主要受细胞因子、细胞特异性转录因子等调控。近年来研究发现,DNA甲基化、组蛋白修饰以及染色质重塑等表观遗传学修饰能调控相关基因的表达,使Th细胞分化成稳定的亚群,发挥相应的功能。同时,这些表观遗传学修饰机制在赋予Th细胞亚群可遗传性、可塑性等方面也发挥着重要的作用。     

Direct helper T cell-induced B cell differentiation involves interaction between T cell antigen CD28 and B cell activation antigen B7.

Cognate interactions between major histocompatibility complex class II antigen (Ag)-reactive CD4+ T helper (Th) and Ag-presenting B cells induce first the activation of B cells and their subsequent differentiation into Ig-secreting cells (IgSC). The Th cell-associated homodimeric glycoprotein CD28 has been implicated as an important regulator of Th activation. Recently, B cell-associated early activation Ag B7 has been identified as a ligand for the CD28 molecule. In this study, we have examined using monoclonal antibodies (mAb) the roles of CD28 and B7 molecules during the Th-B cell cognate interactions leading to the differentiation of B7+ B cells. Anti-CD28 mAb 9.3 specifically inhibited proliferative responses of CD4+ T cells to both allogeneic B cells and soluble Ag-presenting autologous non-T cells. In addition, anti-CD28 mAb 9.3 inhibited Th-induced differentiation of alloantigen-presenting B cells into ISC. Similar inhibition of both Ag-induced Th activation and B cell differentiation into ISC was observed using mAb BB1 which recognizes a B cell-associated molecule B7. In contrast, non-cognate Th-independent exogenous interleukin 6-induced differentiation of B7+ B cells into ISC was not inhibited by mAb to either molecule. These results clearly demonstrate the involvement of CD28 on Th and its ligand B7 on B cells during cognate Th-B interactions leading to the differentiation of B cells. Furthermore, these results also suggest the development of new mAb-based therapeutic approaches for exaggerated B cell activation associated with certain autoimmune diseases such as systemic lupus erythematosus.     

Sendai virus glycoproteins are T cell-dependent B cell mitogens

UV-inactivated Sendai virus is mitogenic for murine splenocytes, whereas infectious Sendai virus kills spleen cells in vitro. The isolated hemagglutinin-neuraminidase (HN) and fusion (F) glycoproteins of Sendai virus are also mitogenic for cultured mouse spleen cells. A mixture of these glycoproteins (1 microgram/well) gives maximum stimulation 96 h after culture initiation. Viral proteins remaining insoluble after Triton X-100 extraction are also mitogenic for mouse spleen cells, with maximum stimulation occurring at 72 h after culture initiation with 1 to 5 microgram/well. On the basis of protein concentration, the HN and F glycoproteins are approximately three times more mitogenic than the Triton X-100-insoluble material. The mitogenic response of the HN and F glycoproteins has two components, a T cell-independent B cell proliferation, which is less than one-half of the total stimulation observed, and a T cell-dependent B cell proliferation. In contrast, the Triton X-100-insoluble material is a T cell-dependent B cell mitogen. Purified T lymphocytes do not respond to the mitogenic signal of either HN-F or Triton X-100-insoluble material.     

Absence of CD40-CD40 ligand interactions in X-linked hyper-IgM syndrome does not affect differentiation of T helper cell subsets

The aim of this study was to investigate the effect of absent CD40-CD40 ligand interactions in patients with X-linked hyper-IgM syndrome (XHIGM) on the generation of Th1 and Th2 immunity. Whole blood from patients and sex- and age-matched controls was stimulated with phorbol myristate acetate (PMA) and calcium ionophore A23187 in the presence of Brefeldin A. After 5 h, cellular production of interferon-gamma, IL-4, tumour necrosis factor-alpha and IL-2 was measured by intracellular cytokine staining and flow cytometry. This method has been shown previously to preferentially activate memory T cells and in preliminary experiments cells making these cytokines were found to be predominantly CD45RO+. No differences in the proportion of T cells (CD3+) or T cell subsets (CD4+/CD8+) secreting these cytokines between XHIGM patients and age- and sex-matched controls were observed. In addition, production of IL-12 and IL-6 by monocytes in response to lipopolysaccharide and CD40 stimulation was equivalent in patients and controls. These results suggest that development of Th1 or Th2 memory cells in patients with XHIGM is unaffected by the absence of functional CD40 ligand. Rather, the susceptibility of these patients to intracellular pathogens, such as Pneumocystis carinii and Cryptosporidium parvum, is more likely to be due to an inability to activate the effector arm of the cellular immune response.     

Cognate interactions between helper T cells and B cells

The mechanism by which mammals produce an antibody response after exposure to antigen has intrigued biologists for over a hundred years. Here, Randolph Noelle and Charles Snow review some of the experimental findings since the early 1970s that have advanced understanding of the mechanisms operating during B-cell activation by thymus-dependent (TD) antigens. They also propose a model for B-cell activation that emphasizes the critical role played by direct cellular interactions between B cells and helper T(TH) cells and seek to place into perspective the role played by the membrane immunoglobulin (mlg) receptor in cognate responses.     

Suppressor T cell induction by soluble OKT3 antibody does not require cross-linking of the T3 molecule

The monoclonal antibody OKT3 can induce T suppressor cells of the in vitro antibody response when used at concentrations as low as 10 ng/ml and in cultures containing as few as 4% OKT3-pretreated T cells. These suppressor cells are radioresistant, cyclosporin sensitive and belong to the T4 as well as the T8 subpopulations. In contrast with the mitogenic effect of OKT3 the suppressor activity can be induced and expressed in highly monocyte-depleted cultures. Moreover F(ab')2 fragments of OKT3, although not mitogenic, are able to induce suppressor cells. Thus cross-linking of the T3 molecule (such as that realized when intact OKT3 IgG and monocytes are present) is not required to induce suppressor cells among resting normal T cells.