T-dependent Responses Research Articles

Induction of HIV-1 Broad Neutralizing Antibodies in 2F5 Knock-in Mice: Selection against Membrane Proximal External Region–Associated Autoreactivity Limits T-Dependent Responses

A goal of HIV-1 vaccine development is to elicit broadly neutralizing Abs (BnAbs). Using a knock-in (KI) model of 2F5, a human HIV-1 gp41 membrane proximal external region (MPER)-specific BnAb, we previously demonstrated that a key obstacle to BnAb induction is clonal deletion of BnAb-expressing B cells. In this study of this model, we provide a proof-of-principle that robust serum neutralizing IgG responses can be induced from pre-existing, residual, self-reactive BnAb-expressing B cells in vivo using a structurally compatible gp41 MPER immunogen. Furthermore, in CD40L-deficient 2F5 KI mice, we demonstrate that these BnAb responses are elicited via a type II T-independent pathway, coinciding with expansion and activation of transitional splenic B cells specific for 2F5's nominal gp41 MPER-binding epitope (containing the 2F5 neutralization domain ELDKWA). In contrast, constitutive production of nonneutralizing serum IgGs in 2F5 KI mice is T dependent and originates from a subset of splenic mature B2 cells that have lost their ability to bind 2F5's gp41 MPER epitope. These results suggest that residual, mature B cells expressing autoreactive BnAbs, like 2F5 as BCR, may be limited in their ability to participate in T-dependent responses by purifying selection that selectively eliminates reactivity for neutralization epitope-containing/mimicked host Ags.

Plasticity is the differentiated state of CD4 T cells

[Extract]The recent application of targeted mutation technologies to lymphocyte fate mapping1 has facilitated the in vivo analysis of CD4 T cells subsequent to their differentiation. The fate mapping of IL17-producing cells has shown them to be responsive to environmental cues in fungal infection and central nervous system autoimmunity. Now Stockinger's group reports that many IL17-producing CD4⁺ T cells home to intestinal Peyer's patches (PP) and convert to a follicular helper T cell (TFH) phenotype, providing critical support for T-dependent IgA responses.

Immunotoxicity and genotoxicity testing of PLGA-PEO nanoparticles in human blood cell model

A human blood cell model for immunotoxicity and genotoxicity testing was used to measure the response to polylactic-co-glycolic acid (PLGA-PEO) nanoparticle (NP) (0.12, 3, 15 and 75 μg/cm2 exposure in fresh peripheral whole blood cultures/isolated peripheral blood mononuclear cell cultures from human volunteers (n = 9–13). PLGA-PEO NPs were not toxic up to dose 3 μg/cm2; dose of 75 μg/cm2 displays significant decrease in [3H]-thymidine incorporation into DNA of proliferating cells after 4 h (70% of control) and 48 h (84%) exposure to NPs. In non-cytotoxic concentrations, in vitro assessment of the immunotoxic effects displayed moderate but significant suppression of proliferative activity of T-lymphocytes and T-dependent B-cell response in cultures stimulated with PWM > CON A, and no changes in PHA cultures. Decrease in proliferative function was the most significant in T-cells stimulated with CD3 antigen (up to 84%). Cytotoxicity of natural killer cells was suppressed moderately (92%) but significantly in middle-dosed cultures (4 h exposure). On the other hand, in low PLGA-PEO NPs dosed cultures, significant stimulation of phagocytic activity of granulocytes (119%) > monocytes (117%) and respiratory burst of phagocytes (122%) was recorded. Genotoxicity assessment revealed no increase in the number of micronucleated binucleated cells and no induction of SBs or oxidised DNA bases in PLGA-PEO-treated cells. To conclude on immuno- and genotoxicity of PLGA-PEO NPs, more experiments with various particle size, charge and composition need to be done.

Jet Fuel Kerosene is not Immunosuppressive in Mice or Rats Following Inhalation for 28 Days

Previous reports indicated that inhalation of JP-8 aviation turbine fuel is immunosuppressive. However, in some of those studies, the exposure concentrations were underestimated, and percent of test article as vapor or aerosol was not determined. Furthermore, it is unknown whether the observed effects are attributable to the base hydrocarbon fuel (jet fuel kerosene) or to the various fuel additives in jet fuels. The present studies were conducted, in compliance with Good Laboratory Practice (GLP) regulations, to evaluate the effects of jet fuel kerosene on the immune system, in conjunction with an accurate, quantitative characterization of the aerosol and vapor exposure concentrations. Two female rodent species (B6C3F1 mice and Crl:CD rats) were exposed by nose-only inhalation to jet fuel kerosene at targeted concentrations of 0, 500, 1000, or 2000 mg/m3 for 6 h daily for 28 d. Humoral, cell-mediated, and innate immune functions were subsequently evaluated. No marked effects were observed in either species on body weights, spleen or thymus weights, the T-dependent antibody-forming cell response (plaque assay), or the delayed-type hypersensitivity (DTH) response. With a few exceptions, spleen cell numbers and phenotypes were also unaffected. Natural killer (NK) cell activity in mice was unaffected, while the NK assessment in rats was not usable due to an unusually low response in all groups. These studies demonstrate that inhalation of jet fuel kerosene for 28 d at levels up to 2000 mg/m3 did not adversely affect the functional immune responses of female mice and rats.

Cell type specific role of B7 for Ag-specific T and B cell activation and differentiation in T-dependent antibody response in vivo (P1017)

Abstract B7.1/B7.2 molecules are essential for eliciting immune responses through CD28-mediated T cell activation, as well as for CTLA-4-mediated negative regulation of T cell responses. Although many cell types (DC, MΦ, B and T) express B7 in vivo, the role of B7 on each cell type is not well defined. Utilizing cell type-specific B7cKO mice, cell type specific roles of B7 for T-dependent GC and Ab responses were analyzed. Upon NP-OVA/Alum immunization, NP-specific IgG1 production was profoundly defective in mice lacking B7 expression on DC (DC-B7KO) but was intact in mice lacking B7 on B cells (B-B7KO). Consistent with this, NP-binding GC B cells and Ag-specific CD4+ T cell expansion were not observed in DC-B7KO. Surprisingly, however, GC formation and total numbers of GC B and TFH were undiminished in DC-B7KO; whereas B-B7KO showed reduction of total number of GC B and TFH despite normal number of NP-binding GC B. In addition, NP-specific secondary Ab was absent in DC-B7KO but intact in B-B7KO. These results clearly demonstrate that B7 on DC but not B cells is critical for Ag-specific CD4+ T cell expansion and IgG1 production as well as for secondary Ab responses. In contrast, a distinct pathway dependent upon B7 on B cells but not DC supports apparently Ag-nonspecific GC response. Further studies are ongoing to evaluate cell type-specific roles of B7 for affinity maturation, memory T/B formation, and potentially nonspecific or autoreactive components during T-dependent Ab response.

Nck proteins are recruited directly to the BCR and regulate PI3K signaling thereby shaping B cell immune responses (P1154)

Abstract Binding of antigen to the B cell antigen receptor (BCR) triggers the activation of different kinases and the initiation of many signaling pathways required for mounting B-dependent immune responses. Scaffolding proteins, recruited downstream of the BCR, act as signal integrators through the recruitment of multiple signaling effectors. Nck is a family of scaffolding proteins best known for linking phosphotyrosine signals to cytoskeleton regulation but until now uncharacterized in the context of B lymphocytes. In our study, Nck was found to be directly recruited to the Igα tail of the BCR and to be important to regulate immune-receptor signaling. Through the generation of a novel Nck1 and Nck2 B cell conditional KO mouse model, we found these adaptors to be essential for mounting T-independent immune response and differentiate into extrafollicular plasma cells during T-dependent responses. The phenotype reported was shown to be due to defective PI3K signaling and Foxo1 downregulation in response to antigen challenge. Our work therefore provides the first compelling evidence for a critical role of Nck in BCR signaling, which is independent of its cytoskeleton regulation yet essential to mount B cell responses.

IL-4 upregulates Igα and Igβ protein, resulting in augmented IgM maturation and amplified BCR-triggered B cell activation in vitro and in vivo (P6230)

Abstract Interleukin 4 (IL-4) is critical for optimal B cell activation in T-dependent responses; however, the underlying mechanism remains elusive. In the present study, we demonstrate that IL-4-pretreatment markedly up-regulates STAT6-dependent Igα and Igβ protein expression in a translational manner in primary B cells. Elevated Igα and Igβ protein significantly promotes IgM maturation and IgM surface expression as evidenced by isolated overexpression of Igα and Igβ in a B cell line and this increased surface IgM is associated with augmented BCR-initiated ERK phosphorylation. In vivo, Tfh-derived IL-4 is critical for germinal center B cell expansion as shown by significant reduction of germinal center B cells in SRBC-immunized IL-4 receptor alpha deficient mice relative to wild type control mice. We find that IL-4 facilitates BCR-initiated pre-germinal center B cell activation by up-regulating Igα, Igβ, surface IgM expression and BCR-triggered pERK, all of which were reversed by administration of neutralizing anti-IL-4 antibody. In sum, this study elucidates a novel mechanism for crosstalk between the IL-4 receptor and B cell receptor.

SAP gene transfer restores cellular and humoral immune function in a murine model of X-linked lymphoproliferative disease

AbstractX-linked lymphoproliferative disease (XLP1) arises from mutations in the gene encoding SLAM-associated protein (SAP) and leads to abnormalities of NKT-cell development, NK-cell cytotoxicity, and T-dependent humoral function. Curative treatment is limited to allogeneic hematopoietic stem cell (HSC) transplantation. We tested whether HSC gene therapy could correct the multilineage defects seen in SAP−/− mice. SAP−/− murine HSCs were transduced with lentiviral vectors containing either SAP or reporter gene before transplantation into irradiated recipients. NKT-cell development was significantly higher and NK-cell cytotoxicity restored to wild-type levels in mice receiving the SAP vector in comparison to control mice. Baseline immunoglobulin levels were significantly increased and T-dependent humoral responses to NP-CGG, including germinal center formation, were restored in SAP-transduced mice. We demonstrate for the first time that HSC gene transfer corrects the cellular and humoral defects in SAP−/− mice providing proof of concept for gene therapy in XLP1.

IMMUNOLOGICAL RESPONSE IN BOVINE LYMPH NODES STIMULATED WITH SUBUNITS VACCINES

The vaccination process belongs to the public health intervention methodologies that help prevent infections. Vaccinations performed successfully in the history of medicine reported the significance of this procedure to increase the quality of life, prevent zoonoses and improve animal production. Vaccine emergence remained without exact rules for a long time, maintaining a close relationship with pathogens. However, subunit vaccines, with a difference from the classical idea of protective immunity with microorganisms showed it is possible to trigger T-dependent responses with peptide, revealing new rules for vaccine development. This vaccination process starts by the modulation chance of adaptive immune response through peptide sequences process by APCs for immune synapse formation interceded for pMHC-TCR as a scaffold to T cells priming. In this way the immunological signal triggered by immune synapses is amplified in lymph nodes. As a consequence, T and B cells modulated by peptide activity interact between the B cell follicles region and T cell aggregates, which constitute the paracortical region of secondary lymphoid tissue to form connate unions as a prerequisite for clonal amplification and subsequent immunological memory. Indicating the knowledge of the mechanisms of immune response generated by peptides immunization is essential for understanding modulation, amplification and immune protection as demands for good subunits vaccine.

Basophils control T cell Responses and limit Disease Activity in Experimental Murine Colitis

Basophils control T cell Responses and limit Disease Activity in Experimental Murine Colitis

Default in plasma and intestinal IgA responses during acute infection by simian immunodeficiency virus

BackgroundConflicting results regarding changes in mucosal IgA production or in the proportions of IgA plasma cells in the small and large intestines during HIV-infection have been previously reported. Except in individuals repeatedly exposed to HIV-1 but yet remaining uninfected, HIV-specific IgAs are frequently absent in mucosal secretions from HIV-infected patients. However, little is known about the organization and functionality of mucosal B-cell follicles in acute HIV/SIV infection during which a T-dependent IgA response should have been initiated. In the present study, we evaluated changes in B-cell and T-cell subsets as well as the extent of apoptosis and class-specific plasma cells in Peyer’s Patches, isolated lymphoid follicles, and lamina propria. Plasma levels of IgA, BAFF and APRIL were also determined.ResultsPlasma IgA level was reduced by 46% by 28 days post infection (dpi), and no IgA plasma cells were found within germinal centers of Peyer’s Patches and isolated lymphoid follicles. This lack of a T-dependent IgA response occurs although germinal centers remained functional with no sign of follicular damage, while a prolonged survival of follicular CD4+ T-cells and normal generation of IgG plasma cells is observed. Whereas the average plasma BAFF level was increased by 4.5-fold and total plasma cells were 1.7 to 1.9-fold more numerous in the lamina propria, the relative proportion of IgA plasma cells in this effector site was reduced by 19% (duodemun) to 35% (ileum) at 28 dpi.ConclusionOur data provide evidence that SIV is unable to initiate a T-dependent IgA response during the acute phase of infection and favors the production of IgG (ileum) or IgM (duodenum) plasma cells at the expense of IgA plasma cells. Therefore, an early and generalized default in IgA production takes place during the acute of phase of HIV/SIV infection, which might impair not only the virus-specific antibody response but also IgA responses to other pathogens and vaccines as well. Understanding the mechanisms that impair IgA production during acute HIV/SIV infection is crucial to improve virus-specific response in mucosa and control microbial translocation.

IP4 regulates FasL-mediated T lymphocyte death via inhibition of Orai1 (115.1)

Abstract Antigen receptor-mediated production of Ins(1,4,5)P3 induces the release of Ca2+ from intracellular stores, resulting in the opening of store-operated Ca2+ (SOC) channels. We previously reported a mouse line deficient in Inositol(1,4,5)P3-3 kinase B (ITPKb), which converts Ins(1,4,5)P3 (IP3) to Ins(1,3,4,5)P4 (IP4). Mice lacking ITPKb exhibit a complete block in T cell positive selection and possess impaired B cell development and activation. Interestingly, ITPKb-/- cells exhibit enhanced Ag receptor-induced SOC signaling, which can be rescued by adding cell-permeable IP4 to ITPKb-/- cells, suggesting that IP4 serves to inhibit SOC channels. Due to developmental blocks, the role of ITPKb in mature lymphocyte function is unknown. Using the ER-Cre-loxP system, tamoxifen-induced deletion of ITPKb (ITPKbfl/fl) demonstrates a crucial role for ITPKb in mature T cell function. While ITPKbfl/fl mice have normal T-independent Ab responses, T-dependent Ab responses are completely abolished. In vitro, mature ITPKbfl/fl T cells exhibit enhanced SOC entry and reduced proliferative responses to TCR signals. Furthermore, we found that ITPKbfl/fl T cells die rapidly after activation, due in part to enhanced FasL upregulation. Using a HEK293 cell line stably expressing Stim1 and Orai1, we demonstrate that IP4 is an inhibitor of open-state Orai1 channels. These data identify ITPKb and IP4 as essential mediators of lymphocyte development and T cell activation by regulating SOC entry via Orai1.

Cell type specific role of B7 for T-dependnet antibody response in vivo (176.14)

Abstract B7.1/B7.2 molecules are essential for eliciting immune responses through CD28-mediated T cell activation, as well as for CTLA-4-mediated negative regulation of T cell responses. Although many cell types (DC, MΦ, B cells and T cells) express B7 in vivo, the role of B7 on each cell type for immune response is not well defined. To address these questions, we generated and characterized cell type-specific B7 cKO mice. Mice expressing a B7.1flox BAC Tg on a B7.1/B7.2 DKO background (designated as B7flox) were generated and crossed to several Cre-Tg strains. To evaluate the cell type-specific role of B7 for T-dependent Ab response in vivo, B7 cKO mice were immunized with TNP-KLH/Alum, and TNP-specific Ab was measured. Ag-specific IgG1 production was B7-dependent, as evidenced by the absence of response in B7 DKO mice. Normal levels of IgG1 production were observed in B7flox mice and in B cell-specific B7 cKO (CD19-Cre x B7flox) mice. In contrast, the IgG1 response was essentially absent in DC-specific B7 cKO (CD11c-Cre x B7flox) mice. This result indicates that B7 on DC is critical for induction of IgG1 class switching, whereas B7 on B cells is not required for this response. Further studies are ongoing to elucidate cell type-specific roles of B7 for several critical steps involved in T-dependent antibody responses, including follicular helper T cell differentiation and germinal center formation, using a panel of cell type-specific B7 cKO mice.

T‐follicular helper cell differentiation and the co‐option of this pathway by non‐helper cells

Human and mouse studies performed over the last decade have established that follicular helper T (Tfh) cells are a CD4(+) helper subset specialized in the provision of help to B cells. Tfh differentiation is driven by expression of the transcriptional repressor B-cell lymphoma-6 (Bcl-6), which turns on a program that guides T cells close to B-cell areas where Tfh cells first provide help to B cells. Sustained Bcl-6 expression promotes the entry of Tfh cells into follicles and modulates their cytokine expression profile so they can support and select germinal center B cells that have acquired affinity-enhancing mutations in their immunoglobulin genes. Forkhead box 3 protein (Foxp3)(+) regulatory T cells and invariant natural killer T (NKT) cells can also co-opt the Bcl-6-dependent follicular differentiation pathway to migrate into B-cell follicles and regulate antibody responses. The resulting NKT follicular helper cells drive a distinctive type of T-dependent B-cell response to lipid-containing antigens, whereas FoxP3(+) follicular regulatory (Tfr) cells exert a suppressive function on germinal centers. Elucidating how Tfr cells are functionally and numerically regulated and the factors that control the balance between Tfh and Tfr cells is likely to be critical for improved understanding of the pathogenesis and progression of autoimmunity and lymphomas of germinal center origin, and generation of effective vaccines.

Pneumococcal Conjugate and Plain Polysaccharide Vaccines Have Divergent Effects on Antigen-Specific B Cells

Background. A 23-valent unconjugated pneumococcal polysaccharide vaccine (23vP), routinely administered at the age of 65, has limited effectiveness, and revaccination induces attenuated antibody responses. It is not known whether pneumococcal polysaccharide-protein conjugated vaccines (PCV), although highly effective in infants, offer any immunological advantages over 23vP in adults.Methods. We immunized adults with schedules combining both PCV and 23vP and investigated B-cell responses to establish whether PCV7 (a 7-valent PCV) induced T-dependent responses in adults, to assess the role of memory B cells in 23vP-induced antibody hyporesponsiveness, and to identify the B-cell subtypes involved.Results. A single dose of PCV7 induced significant increases in serotype-specific memory B-cell populations in peripheral blood indicating a T-dependent response. Conversely, immunization with 23vP resulted in a decrease in memory B-cell frequency. Furthermore, memory B-cell responses to subsequent immunization with PCV7, when given after 23vP, were attenuated. Notably, B1b cells, a subset important in protecting mice against pneumococci, were also depleted following immunization with 23vP in humans.Conclusions. This study indicates that PCV7 may have an immunological advantage over 23vP in adults and that 23vP-induced depletion of memory and B1b-cell subsets may provide a basis for antibody hyporesponsiveness and the limited effectiveness of 23vP.Clinical Trials Registration. ISRCTN: 78768849.

B cells regulate antibody responses through the medullary remodeling of inflamed lymph nodes

Lymph node (LN) structure is remodeled during immune responses, a process which is considered to play an important role in the regulation of immune function. To date, little attention has been paid to the remodeling of the medullary region, despite its proposed role as a niche for antibody-producing plasma cells. Here, we show that B cells mediate medullary remodeling of antigen-draining LNs during inflammation. This process occurs with kinetics similar to changes in plasma cell number and is accompanied by stromal renetworking which manifests as the segregation of B cells and plasma cells. Medullary remodeling depends on signaling via the lymphotoxin-β receptor and the presence of B cells but occurs independently of T-dependent humoral responses or other immune cell subsets including T cells, monocytes and neutrophils. Moreover, reconstitution of non-cognate polyclonal B cells in B cell-deficient mice restores not only the medullary remodeling but also the antibody response by separately transferred cognate B cells, suggesting that non-cognate B cells contribute to antibody responses through medullary remodeling. We propose that non-cognate B cells mediate the expansion of the plasma cell niche in LN through medullary remodeling, thereby regulating the size of the LN plasma cell pool.

CD40L-null NKT cells provide B cell help for specific antibody responses

CD1d-binding glycolipids exert potent adjuvant effects on T-dependent Ab responses. The mechanisms include cognate interaction between CD1d-expressing B cells and TCR-expressing Type I CD1d-restricted natural killer T cells (NKTs). However, the critical NKT-derived factors that stimulate B cells are poorly understood. We tested the hypothesis that CD1d-driven CD40L expression by NKT cells influences humoral immunity. Bone marrow chimeras with CD40L(+/+) or CD40L(-/-) NKT cells were immunized with Ag plus CD1d ligand before measuring Ab responses. CD40L(-/-) NKT cells stimulated higher endpoint Ab titers than controls expressing CD40L. In contrast, immunization of CD40L(-/-) mice revealed that CD40L(-/-) NKT cells could not provide B cell help when Th cells lacked CD40L. We report that CD40L(-/-) NKT cells can provide help for Ab production and do so cooperatively with CD40L(+/+) Th cells. We suggest that the manner in which NKT cells provide B cell help is distinct from that of Th cells.

EBI2 Guides Serial Movements of Activated B Cells and Ligand Activity Is Detectable in Lymphoid and Nonlymphoid Tissues

EBV-induced gene 2 (EBI2) was recently shown to direct the delayed movement of activated B cells to interfollicular and outer follicular regions of secondary lymphoid organs and to be required for mounting a normal T-dependent Ab response. In this study, we show that EBI2 promotes an early wave of Ag-activated B cell migration to the outer follicle in mice. Later, when B cells have moved to the T zone in a CCR7-dependent manner, EBI2 helps distribute the cells along the B zone-T zone boundary. Subsequent EBI2-dependent movement to the outer follicle coincides with CCR7 downregulation and is promoted by CD40 engagement. Using a bioassay, we identify a proteinase K-resistant, hydrophobic EBI2 ligand activity in lymphoid and nonlymphoid tissues. Production of EBI2 ligand activity by a cell line is sensitive to statins, suggesting production in a 3-hydroxy-3-methyl-glutaryl-CoA reductase-dependent manner. CD40-activated B cells show sustained EBI2-dependent responsiveness to the bioactivity. These findings establish a role for EBI2 in helping control B cell position at multiple stages during the Ab response and they suggest that EBI2 responds to a broadly distributed lipid ligand.

Immune evaluation and vaccine responses in Down syndrome: Evidence of immunodeficiency?

BackgroundPatients with Down syndrome (DS) appear to be at a greater risk for serious infections, but it is unclear whether this is due to anatomic variations or intrinsic immune defects. ObjectiveWe assessed a cohort of pediatric subjects with DS to determine if immunological abnormalities indeed account for the excess infections. MethodsWe performed quantitative assessment of T-independent (type 2 – pneumococcal polysaccharide vaccine) and T-dependent Ab responses (with inactivated seasonal influenza vaccine) along with numerical quantitation of lymphocyte subpopulations and thymic output in a random population sample of children with DS (cases) along with family-matched sibling or community controls. ResultsMedian serum IgG levels were significantly higher in cases (1090mg/dL) as compared with controls (808mg/dL, P=0.02). Cases had significantly lower median CD4 T cell counts than the controls (636 cells/μL, P=0.01). Cases had reduced CD19 B cell counts and CD19% than the controls (P=0.009 and 0.006 respectively). Cases also showed decreased total memory (CD19+CD27+, P=0.002) and class-switched memory (CD19+CD27+IgM−IgD−, P=0.004) B cells.The median CD4 recent thymic emigrant (RTE) in females and males cases was lower than controls (P=0.007 and 0.07 respectively). Cases had a lower median T cell receptor excision circle (TREC) count of 2556 as compared to the controls count of 5216, P<0.006 although both the cases and controls were within the established reference range. There were no differences in the percentage of cases and controls who responded to inactivated influenza vaccine, but the response to polysaccharide pneumococcal vaccine was suboptimal in cases. ConclusionsOur study suggests that there are subtle abnormalities in both humoral and cellular arms of the immune response in children with DS as compared to the control subjects.

Rapid Reconstitution of Antibody Responses Following Transplantation of Purified Allogeneic Hematopoietic Stem Cells

Allogeneic hematopoietic cell transplantation has broad clinical applications extending from the treatment of malignancies to induction of immunologic tolerance. However, adaptive cellular and humoral immunity frequently remain impaired posttransplantation. Here, recovery of T-dependent and T-independent Ab responses was evaluated in mice transplanted with purified hematopoietic stem cells (HSCs) devoid of the mature immune cells believed to hasten immune recovery. Mixed and full donor chimeras were created by conditioning recipients with sublethal or lethal irradiation, respectively, across different donor/host genetic disparities. By 6 wk posttransplantation, all animals demonstrated robust T-independent Ab responses, and all mixed chimeras and recipients of MHC-matched or haploidentical HSCs with a shared MHC haplotype had T-dependent Ab responses equivalent to those of untransplanted controls. Full chimeras that received fully MHC-disparate HSCs showed delayed T-dependent Ab responses that recovered by 12 wk. This delay occurred despite early reconstitution and proper migration to germinal centers of donor-derived T(follicular helper) (T(FH)) cells. Congenic transplants into T(FH)-deficient CD4(-/-) mice revealed restoration of T-dependent Ab responses by 6 wk, leading us to conclude that MHC disparity caused delay in humoral recovery. These findings, together with our previous studies, show that, contrary to the view that depletion of graft lymphocytes results in poor posttransplant immunity, elimination of immune-suppressing graft-versus-host reactions permits superior immune reconstitution. This study also provides insight into the regeneration of T(FH) cells and humoral immunity after allogeneic HSC transplantation.