Amanda L. Gavin

Adjuvant-Enhanced Antibody Responses in the Absence of Toll-Like Receptor Signaling

Innate immune signals mediated by Toll-like receptors (TLRs) have been thought to contribute considerably to the antibody-enhancing effects of vaccine adjuvants. However, we report here that mice deficient in the critical signaling components for TLR mount robust antibody responses to T cell–dependent antigen given in four typical adjuvants: alum, Freunds complete adjuvant, Freunds incomplete adjuvant, and monophosphoryl-lipid A/trehalose dicorynomycolate adjuvant. We conclude that TLR signaling does not account for the action of classical adjuvants and does not fully explain the action of a strong adjuvant containing a TLR ligand. This may have important implications in the use and development of vaccine adjuvants.

FcγRI-Deficient Mice Show Multiple Alterations to Inflammatory and Immune Responses

The inactivation of the mouse high-affinity IgG Fc receptor FcgammaRI resulted in a wide range of defects in antibody Fc-dependent functions. These studies showed the primary importance of FcgammaRI in endocytosis of monomeric IgG, kinetics, and extent of phagocytosis of immune complexes, in macrophage-based ADCC, and in immune complex-dependent antigen presentation to primed T cells. In the absence of FcgammaRI, antibody responses were elevated, implying the removal of a control point by the deletion of FcgammaRI. In addition, FcR-gamma chain-deficient mice were found to express partially functional FcgammaRI. Thus, FcgammaRI is an early participant in Fc-dependent cell activation and in the development of immune responses.

Metabolic Reprogramming Is Required for Antibody Production That Is Suppressed in Anergic but Exaggerated in Chronically BAFF-Exposed B Cells

B cell activation leads to proliferation and Ab production that can protect from pathogens or promote autoimmunity. Regulation of cell metabolism is essential to support the demands of lymphocyte growth and effector function and may regulate tolerance. In this study, we tested the regulation and role of glucose uptake and metabolism in the proliferation and Ab production of control, anergic, and autoimmune-prone B cells. Control B cells had a balanced increase in lactate production and oxygen consumption following activation, with proportionally increased glucose transporter Glut1 expression and mitochondrial mass upon either LPS or BCR stimulation. This contrasted with metabolic reprogramming of T cells, which had lower glycolytic flux when resting but disproportionately increased this pathway upon activation. Importantly, tolerance greatly affected B cell metabolic reprogramming. Anergic B cells remained metabolically quiescent, with only a modest increase in glycolysis and oxygen consumption with LPS stimulation. B cells chronically stimulated with elevated BAFF, however, rapidly increased glycolysis and Ab production upon stimulation. Induction of glycolysis was critical for Ab production, as glycolytic inhibition with the pyruvate dehydrogenase kinase inhibitor dichloroacetate sharply suppressed B cell proliferation and Ab secretion in vitro and in vivo. Furthermore, B cell–specific deletion of Glut1 led to reduced B cell numbers and impaired Ab production in vivo. Together, these data show that activated B cells require Glut1-dependent metabolic reprogramming to support proliferation and Ab production that is distinct from T cells and that this glycolytic reprogramming is regulated in tolerance.

ΔBAFF, a Splice Isoform of BAFF, Opposes Full-Length BAFF Activity In Vivo in Transgenic Mouse Models

ΔBAFF is a novel splicing isoform of the regulator B cell-activating factor (BAFF, BLyS), a TNF family protein with powerful immunoregulatory effects. Overexpression of BAFF leads to excessive B cell accumulation, activation, autoantibodies, and lupus-like disease, whereas an absence of BAFF causes peripheral B cell immunodeficiency. Based on the ability of ΔBAFF to multimerize with full-length BAFF and to limit BAFF proteolytic shedding from the cell surface, we previously proposed a role for ΔBAFF in restraining the effects of BAFF and in regulating B lymphocyte homeostasis. To test these ideas we generated mice transgenic for ΔBAFF under the control of human CD68 regulatory elements, which target expression to myeloid and dendritic cells. We also generated in parallel BAFF transgenic mice using the same expression elements. Analysis of the transgenic mice revealed that ΔBAFF and BAFF had opposing effects on B cell survival and marginal zone B cell numbers. ΔBAFF transgenic mice had reduced B cell numbers and T cell-dependent Ab responses, but normal preimmune serum Ig levels. In contrast, BAFF transgenic mice had extraordinarily elevated Ig levels and increases in subsets of B cells. Unexpectedly, both BAFF and ΔBAFF appeared to modulate the numbers of B-1 phenotype B cells.

ΔBAFF, an Alternate Splice Isoform That Regulates Receptor Binding and Biopresentation of the B Cell Survival Cytokine, BAFF

The tumor necrosis family member BAFF is limiting for the survival of follicular B lymphocytes, but excessive BAFF signaling can lead to autoimmunity, suggesting that its activity must be tightly regulated. We have identified a conserved alternate splice isoform of BAFF, called ΔBAFF, which lacks 57 nt encoding the A–A1 loop and is co-expressed with BAFF in many mouse and human myeloid cells. Mouse ΔBAFF appears on the plasma membrane, but unlike BAFF it is inefficiently released by proteolysis. ΔBAFF can associate with BAFF in heteromultimers and diminish BAFF bioactivity and release. Thus, alternative splicing of the BAFF gene suppresses BAFF B cell stimulatory function in several ways, and ΔBAFF may promote other functions as well.

Regulation of the B Cell Receptor Repertoire and Self-Reactivity by BAFF

The TNF-family cytokine BAFF (BLyS) promotes B lymphocyte survival and is overexpressed in individuals with systemic lupus erythematosus and Sjögrens Syndrome. BAFF can rescue anergic autoreactive B cells from death, but only when competition from nonautoreactive B cells is lacking. Yet, high BAFF levels promote autoantibody formation in individuals possessing diverse B cells. To better understand how excess BAFF promotes autoimmunity in a polyclonal immune system, Ig L chain usage was analyzed in 3H9 site-directed IgH chain transgenic mice, whose B cells recognize DNA and chromatin when they express certain endogenous L chains. BAFF levels were manipulated in 3H9 mice by introducing transgenes expressing either BAFF or its natural inhibitor ΔBAFF. B cells in BAFF/3H9 mice were elevated in number, used a broad L chain repertoire, including L chains generating high-affinity autoreactivity, and produced abundant autoantibodies. Comparison of spleen and lymph node B cells suggested that highly autoreactive B cells were expanded. By contrast, ΔBAFF/3H9 mice had reduced B cell numbers with a repertoire similar to that of 3H9 mice, but lacking usage of a subset of Vκ genes. The results show that limiting BAFF signaling only slightly selects against higher affinity autoreactive B cells, whereas its overexpression leads to broad tolerance escape and positive selection of autoreactive cells. The results have positive implications for the clinical use of BAFF-depleting therapy.

Repertoire-based selection into the marginal zone compartment during B cell development

Marginal zone (MZ) B cells resemble fetally derived B1 B cells in their innate-like rapid responses to bacterial pathogens, but the basis for this is unknown. We report that the MZ is enriched in “fetal-type” B cell receptors lacking N regions (N−). Mixed bone marrow (BM) chimeras, made with adult terminal deoxynucleotidyl transferase (TdT)+/+ and TdT−/− donor cells, demonstrate preferential repertoire-based selection of N− B cells into the MZ. Reconstitution of irradiated mice with adult TdT+/+ BM reveals that the MZ can replenish N− B cells in adult life via repertoire-based selection and suggest the possibility of a TdT-deficient precursor population in the adult BM. The mixed chimera data also suggest repertoire-based bifurcations into distinct BM and splenic maturation pathways, with mature “recirculating” BM B cells showing a very strong preference for N+ complementarity-determining region (CDR) 3 compared with follicular B cells. Because the T1 and MZ compartments are both the most enriched for N− H-CDR3, we propose a novel direct T1→MZ pathway and identify a potential T1–MZ precursor intermediate. We demonstrate progressive but discontinuous repertoire-based selection throughout B cell development supporting multiple branchpoints and pathways in B cell development. Multiple differentiation routes leading to MZ development may contribute to the reported functional heterogeneity of the MZ compartment.

An immunoglobulin Cκ-reactive single chain antibody fusion protein induces tolerance through receptor editing in a normal polyclonal immune system

Understanding immune tolerance mechanisms is a major goal of immunology research, but mechanistic studies have generally required the use of mouse models carrying untargeted or targeted antigen receptor transgenes, which distort lymphocyte development and therefore preclude analysis of a truly normal immune system. Here we demonstrate an advance in in vivo analysis of immune tolerance that overcomes these shortcomings. We show that custom superantigens generated by single chain antibody technology permit the study of tolerance in a normal, polyclonal immune system. In the present study we generated a membrane-tethered anti-Igκ–reactive single chain antibody chimeric gene and expressed it as a transgene in mice. B cell tolerance was directly characterized in the transgenic mice and in radiation bone marrow chimeras in which ligand-bearing mice served as recipients of nontransgenic cells. We find that the ubiquitously expressed, Igκ-reactive ligand induces efficient B cell tolerance primarily or exclusively by receptor editing. We also demonstrate the unique advantages of our model in the genetic and cellular analysis of immune tolerance.

Unique Monoclonal Antibodies Define Expression of FcγRI on Macrophages and Mast Cell Lines and Demonstrate Heterogeneity Among Subcutaneous and Other Dendritic Cells

The mouse FcγRI is one of the most fundamentally important FcRs. It participates in different stages of immunity, being a low affinity receptor for T-independent IgG3 and yet a high affinity receptor for IgG2a, the product of a Th1 immune response. However, analysis of this receptor has been difficult due largely to the failure to generate specific Abs to this FcR. We have made use of the polymorphic differences between BALB/c and NOD/Lt mice to generate mAb specific for the FcγRI of BALB/c and the majority of in-bred mouse strains. Three different mAb were obtained that detected FcγRI encoded by the more common Fcgr1a and Fcgr1b alleles, and although they identified different epitopes, none inhibited the binding of IgG to FcγRI. When bound to FcγRI, these mAb induced calcium mobilization upon cross-linking. Several novel observations were made of the cellular distribution of FcγRI. Resting and IFN-γ-induced macrophages expressed FcγRI as well as mast cell lines. Both bone marrow-derived and freshly isolated dendritic cells from spleen and lymph nodes expressed FcγRI. A class of DC, uniquely found in s.c. lymph nodes, expressed the highest level of FcγRI and also high levels of MHC class II, DEC205, CD40, and CD86, with a low level of CD8α, corresponding to the phenotype for Langerhans-derived DC, which are highly active in Ag processing. Thus, in addition to any role in effector functions, FcγRI on APC may act as a link between innate and adaptive immunities by binding and mediating the uptake of T-independent immune complexes for presentation, thereby assisting in the development of T-dependent immune responses.

Protein Kinase C η Is Required for T Cell Activation and Homeostatic Proliferation

Studies of knockout mice delineate specific and overlapping roles for different protein kinase C isoforms in T cell biology. PKC Isoform Specificity in T Cells When a T cell encounters an antigen-presenting cell (APC), protein kinase C θ (PKCθ) is recruited to the immunological synapse, the interface between the T cell and the APC, where it is thought to mediate co-receptor signaling during T cell activation. Although PKCθ is the most abundant PKC isoform in T cells, thymocyte development in PKCθ-deficient mice is only mildly affected. Fu et al. investigated a role for PKCη, which, unlike PKCθ, increases in abundance upon T cell activation. The authors found that similar to PKCθ, PKCη was recruited to the immunological synapse upon T cell activation and that both isoforms had some overlapping functions. However, PKCη was required for antigen-dependent activation, and loss of PKCη, but not that of PKCθ, resulted in decreased homeostatic proliferation of T cells. Together, these data suggest that PKCη plays critical roles in T cells. Protein kinase C η (PKCη) is abundant in T cells and is recruited to the immunological synapse that is formed between a T cell and an antigen-presenting cell; however, its function in T cells is unknown. We showed that PKCη was required for the activation of mature CD8+ T cells through the T cell receptor. Compared with wild-type T cells, PKCη−/− T cells showed poor proliferation in response to antigen stimulation, a trait shared with T cells deficient in PKCθ, which is the most abundant PKC isoform in T cells and was thought to be the only PKC isoform with a specific role in T cell activation. In contrast, only PKCη-deficient T cells showed defective homeostatic proliferation, which requires self-antigen recognition. PKCη was dispensable for thymocyte development; however, thymocytes from mice doubly deficient in PKCη and PKCθ exhibited poor development, indicating some redundancy between the PKC isoforms. Deficiency in PKCη or PKCθ had opposing effects on the relative numbers of CD4+ and CD8+ T cells. PKCη−/− mice had a higher ratio of CD4+ to CD8+ T cells compared to that of wild-type mice, whereas PKCθ−/− mice had a lower ratio. Mice deficient in both isoforms exhibited normal cell ratios. Together, these data suggest that PKCη shares some redundant roles with PKCθ in T cell biology and also performs nonredundant functions that are required for T cell homeostasis and activation.