Marilia Cascalho

Banff '05 Meeting Report: Differential Diagnosis of Chronic Allograft Injury and Elimination of Chronic Allograft Nephropathy (‘CAN’)

The 8th Banff Conference on Allograft Pathology was held in Edmonton, Canada, 15–21 July 2005. Major outcomes included the elimination of the non‐specific term ‘chronic allograft nephropathy’ (CAN) from the Banff classification for kidney allograft pathology, and the recognition of the entity of chronic antibody‐mediated rejection. Participation of B cells in allograft rejection and genomics markers of rejection were also major subjects addressed by the conference.

53BP1 is required for class switch recombination

53BP1 participates early in the DNA damage response and is involved in cell cycle checkpoint control. Moreover, the phenotype of mice and cells deficient in 53BP1 suggests a defect in DNA repair (Ward et al., 2003b). Therefore, we asked whether or not 53BP1 would be required for the efficient repair of DNA double strand breaks. Our data indicate that homologous recombination by gene conversion does not depend on 53BP1. Moreover, 53BP1-deficient mice support normal V(D)J recombination, indicating that 53BP1 is not required for “classic” nonhomologous end joining. However, class switch recombination is severely impaired in the absence of 53BP1, suggesting that 53BP1 facilitates DNA end joining in a way that is not required or redundant for the efficient closing of RAG-induced strand breaks. These findings are similar to those observed in mice or cells deficient in the tumor suppressors ATM and H2AX, further suggesting that the functions of ATM, H2AX, and 53BP1 are closely linked.

Biological implications of cell fusion

Until recently, cells were thought to be integral and discrete components of tissues, and their state was determined by cell differentiation. However, under some conditions, stem cells or their progeny can fuse with cells of other types, mixing cytoplasmic and even genetic material of different (heterotypic) origins. The fusion of heterotypic cells could be of central importance for development, repair of tissues and the pathogenesis of disease.

A quasi-monoclonal mouse.

As a model for studying the generation of antibody diversity, a gene-targeted mouse was produced that is hemizygous for a rearranged V(D)J segment at the immunoglobulin (Ig) heavy chain locus, the other allele being nonfunctional. The mouse also has no functional kappa light chain allele. The heavy chain, when paired with any lambda light chain, is specific for the hapten (4-hydroxy-3-nitrophenyl) acetyl (NP). The primary repertoire of this quasi-monoclonal mouse is monospecific, but somatic hypermutation and secondary rearrangements change the specificity of 20 percent of the antigen receptors on B cells. The serum concentrations of the Ig isotypes are similar to those in nontransgenic littermates, but less than half of the serum IgM binds to NP, and none of the other isotypes do. Thus, neither network interactions nor random activation of a small fraction of the B cell population can account for serum Ig concentrations.

The Immunological Barrier to Xenotransplantation

The authors thank Rob Holzknecht for preparation of the figures and Bekka Pecka and Kim Barber for assistance in preparing the manuscript. Work in the laboratories of the authors on which this communication was in part based is supported by the National Institutes of Health.

Dendritic cells associated with plasmablast survival

A subset of myeloid dendritic cells is described which is associated with the ability of splenic and lymph node plasmablasts to survive and differentiate into plasma cells. Plasmablast‐associated dendritic cells (PDC) are CD11chigh, DEC‐205– and unlike conventional dendritic cells do not associate with T cells. The following findings suggest a requirement for PDC if plasmablasts are to differentiate to plasma cells. First, when large numbers of B cells are recruited into antibody responses and plasmablasts outgrow the PDC stroma, only those associated with PDC survive and differentiate into plasma cells. Conversely, if the number of PDC is increased by ligating their CD40, more plasmablasts survive on the expanded PDC stroma and differentiate into plasma cells. Finally, in T cell‐deficient mice, the plasma cells that develop atypically in the T zones in response to thymus‐independent antigens are associated with ectopic PDC.

Short-lived and Long-lived Bone Marrow Plasma Cells Are Derived from a Novel Precursor Population

The contribution that long-lived bone marrow (BM) plasma cells (PCs) provide to enduring humoral immunity has been underscored by a number of recent studies. However, little is known about the immediate precursors that give rise to long-lived PCs in the BM of immune individuals. We have identified subsets of antigen-experienced B cells within the immune BM that are precursors to PCs. These PC precursors arise in the BM 14 days after immunization and persist for greater than 9 months. Phenotypically distinct subsets of PC precursors give rise to short-lived or long-lived PCs. The differentiation of PC precursors to PCs occurs in the absence of antigen and requires cell division. The functional significance of these newly identified PC precursors in the persistence and quality of the humoral immune response is discussed.

Short-circuiting long-lived humoral immunity by the heightened engagement of CD40

Agonistic alpha CD40 Abs have been shown to be potent immune adjuvants for both cell- and humoral-mediated immunity. While enhancing short-lived humoral immunity, the administration of a CD40 agonist during thymus-dependent immune responses ablates germinal center formation, prematurely terminates the humoral immune response, blocks the generation of B cell memory, and prevents the generation of long-lived bone marrow plasma cells. Interestingly, some of these effects of heightened CD40 engagement could be mimicked by enhancing the magnitude of antigen-specific T cell help. Taken together, these studies demonstrate that as the magnitude of CD40 signaling intensifies, the fate of antigen-reactive B cells can be dramatically altered. These are the first studies to describe the multifaceted function of CD40 in determining the fate of antigen-reactive B cells and provide novel insights into how CD40 agonists can short-circuit humoral immunity.

TACI Is Required for Efficient Plasma Cell Differentiation in Response to T-Independent Type 2 Antigens

The control of systemic infection by encapsulated microorganisms requires T-independent type II (TI-2) Ab responses to bacterial polysaccharides. To understand how such responses evolve, we explored the function of transmembrane activator calcium modulator and cyclophilin ligand interactor (TACI), a member of the TNFR family, required for TI-2 Ab production. Quasimonoclonal (QM) mice produce robust TI-2 responses to 4-hydroxy-3-nitrophenylacetate (NP)-Ficoll, owing to the high precursor frequency of NP-specific B cells in the marginal zone of the spleen. QM mice that lack TACI produce decreased numbers of IgM (2-fold) and IgG (1.6-fold) NP-specific ASCs, compared with TACI-positive QM mice in response to immunization with NP-Ficoll. Our studies indicate that TACI acts at a remote time from activation because TACI is not necessary for activation and proliferation of B cells both in vitro and in vivo. Instead, TACI-deficient QM B cells remained in the cell cycle longer than TACI-proficient QM cells and had impaired plasma cell differentiation in response to NP-Ficoll. We conclude that TACI has dual B cell-autonomous functions, inhibiting prolonged B cell proliferation and stimulating plasma cell differentiation, thus resolving the longstanding paradox that TACI may have both B cell-inhibitory and -stimulatory functions. By promoting plasma cell differentiation earlier during clonal expansion, TACI may decrease the chances of autoantibody production by somatic hypermutation of Ig genes in response to T-independent Ags.

The Double-Edged Sword of Activation-Induced Cytidine Deaminase

Activation-induced cytidine deaminase (AID) is required for Ig class switch recombination, a process that introduces DNA double-strand breaks in B cells. We show in this study that AID associates with the DNA-dependent protein kinase catalytic subunit (DNA-PKcs) promoting cell survival, presumably by resolving DNA double-strand breaks. Wild-type cells expressing AID mutants that fail to associate with DNA-PKcs or cells deficient in DNA-PKcs or 53BP1 expressing wild-type AID accumulate γH2AX foci, indicative of heightened DNA damage response. Thus, AID has two independent functions. AID catalyzes cytidine deamination that originates DNA double-strand breaks needed for recombination, and it promotes DNA damage response and cell survival. Our results thus resolve the paradox of how B cells undergoing DNA cytidine deamination and recombination exhibit heightened survival and suggest a mechanism for hyperIgM type II syndrome associated with AID mutants deficient in DNA-PKcs binding.