Biomedical subjects
Publications and source records attributed to Ian C M MacLennan.
There has been recent progress in understanding the mechanism of somatic hypermutation in germinal centers. By contrast, there is more theory than certainty about the way those cells that acquire heightened affinity for antigen are subsequently selected. Wang and Carter (2005) highlight this in the current issue of Immunity in a study of germinal center development in mice with impaired CD19 signaling.
Naive CD4 T cells develop Th2 activity early in primary responses to alum-precipitated proteins by producing IL-4 mRNA and inducing B cells to produce gamma1 and epsilon switch transcripts. Both IL-4-dependent and IL-4-independent pathways for IL-4 induction are recognized, but their relative contribution to the different phases of primary Th2 responses in vivo is uncertain. We show the primary induction of IL-4 synthesis in lymph nodes responding to alum-precipitated protein is overwhelmingly in antigen-specific CD4 T cells and is unimpaired in IL-4Ralpha(-/-) mice, which can produce but do not respond to IL-4 and IL-13. Ig class-switching in extra-follicular responses, reflecting Th2 activity, is also unimpaired in these mice. By contrast, 7 days after immunization--when T cells are selecting B cells in germinal centers and T cell priming has occurred--non-responsiveness to IL-4 is associated with smaller germinal centers, increased levels of T-bet and gamma2a switch transcripts and reduced gamma1 and epsilon transcripts. These data indicate that Th2 characteristics acquired during T cell priming and the initial CD4 T cell interaction with B cells are largely IL-4-independent, whereas IL-4 production induced during priming has a significant role in maintaining the Th2 phenotype as T cells select B cells in germinal centers.
Features of the Th1 or Th2 phenotype start to develop during CD4 T cell priming. This study of the response to the bacterial flagellar protein FliC shows that either Th1 or Th2 responses can be induced in mice depending upon how FliC is presented. This is shown by assessing the cytokine mRNA and class of FliC-specific plasma cells induced in situ. Soluble recombinant (r)FliC and polymerized FliC are strongly Th2 polarizing, inducing IL-4, NIP45 and c-Maf mRNA as well as epsilon and gamma1 switch transcripts and switching to IgG1. CD28-requirement for this switching shows its T cell dependence. rFliC was unable to induce markers of Th1 activity including IL-12, T-bet and IFN-gamma. Conversely, when FliC is presented in its native context surface-bound on live, flagellated Salmonella, switching is predominantly to IgG2a (IgG2c in C57BL/6 mice), reflecting Th1 activity. The development of divergent FliC-specific polarization to either Th1 or Th2 indicates that the context in which this antigen is encountered rather than its intrinsic immunostimulatory properties determines the direction of Th polarization.
B lymphocyte stimulator (BLyS), a TNF family protein essential for peripheral B cell development, functions primarily through attenuation of B cell apoptosis. In this study, we show that BLyS activates NF-kappaB through both classical and alternative pathways with distinct kinetics in quiescent mature B cells. It rapidly and transiently enhances the p50/p65 DNA binding activity and induces phosphorylation of IkappaBalpha characteristic of the classical NF-kappaB pathway, albeit maintaining IkappaBalpha at a constant level through ongoing protein synthesis and proteasome-mediated destruction. With delayed kinetics, BLyS promotes the processing of p100 to p52 and sustained formation of p52/RelB complexes via the alternative NF-kappaB pathway. p50 is dispensable for p100 processing. However, it is required to mediate the initial BLyS survival signals and concomitant activation of Bcl-x(L) in quiescent mature B cells ex vivo. Although also a target of BLyS activation, at least one of the A1 genes, A1-a, is dispensable for the BLyS survival function. These results suggest that BLyS mediates its survival signals in metabolically restricted quiescent B cells, at least in part, through coordinated activation of both NF-kappaB pathways and selective downstream antiapoptotic genes.
B cells and Abs play a key role in controlling the erythrocytic stage of malaria. However, little is known about the way the humoral response develops during infection. We show that Plasmodium chabaudi chabaudi causes major, but temporary changes in the distribution of leukocytes in the spleen. Despite these changes, an ordered response to infection develops, which includes vigorous extrafollicular growth of plasmablasts and germinal center formation. Early in the response, the lymphocytes in the T zone and follicles become widely spaced, and the edges of these compartments blur. This effect is maximal around the peak of parasitemia. Germinal centers are apparent by day 8, peak at day 20, and persist through day 60. Extrafollicular foci of plasmablasts are visible from day 4 and initiate a very strong plasma cell response. Initially, the plasma cells have a conventional red pulp distribution, but by day 10 they are unconventionally sited in the periarteriolar region of the white pulp. In this region they form clusters occupying part of the area normally filled by T cells. B cells are absent from the marginal zone for at least 30 days after the peak of infection, although flow cytometry shows their continued presence in the spleen throughout infection. Relatively normal splenic architecture is regained by day 60 of infection. These results show that the changes in splenic cell distribution are linked to the presence of parasites and do not seem to interfere with the development of the humoral response.
Antibodies against bacterial capsular polysaccharides play a critical protective role. Responses to these antigens can occur without the help or control of T cells and are associated with marginal zone (MZ) B cells. Capsular antigens are diverse and some cross-react with self-carbohydrate epitopes. This diversity may explain the recruitment of non-autoreactive recirculating B cells and memory B cells to the MZ in addition to other B cells, some of which are weakly autoreactive cells, that are recruited to the MZ without entering the recirculating pool. To test whether memory B cells respond to polysaccharide-based antigens, mice with hapten-specific memory B cells were challenged with hapten-polysaccharide. Hapten-specific plasma cells producing high affinity antibody with Ig V-region mutations were induced. To test whether naive recirculating B cells can form MZ cells that respond to polysaccharide, recirculating B cells from lymph nodes were transferred into Rag-1-deficient mice. MZ cells differentiated from the donor cells without proliferation or T cell help and responded to polysaccharide-based antigen. The differentiation of B cells both from germinal centers and the recirculating pool to the MZ phenotype is likely to make an important contribution to the repertoire of B cells that respond to polysaccharide antigens.
In adaptive antibody responses, B cells are induced to grow either in follicles where they form germinal centers or in extrafollicular foci as plasmablasts. Extrafollicular growth typically occurs in the medullary cords of lymph nodes and in foci in the red pulp of the spleen. It is not a feature of secondary lymphoid tissue associated with the internal epithelia of the body. All types of naïve and memory B cells can be recruited into extrafollicular responses. These responses are associated with immunoglobulin class switching but, at the most, only low-level hypermutation.
Virgin T cells being primed to Th2-inducing or Th1-inducing Ags, respectively, start to synthesize IL-4 or IFN-gamma as they begin to proliferate. Parallel respective induction of B cells to produce gamma1 or gamma2a switch transcripts provides additional evidence of early divergent Th activity. This report concerns the roles of IL-4, IL-13, and B cells in these early events in vivo. Th2 responses were induced in lymph nodes against hapten-protein given s.c. with killed Bordetella pertussis adjuvant. In T cell proliferation in wild-type mice, IL-4 message up-regulation and gamma1 and epsilon switch transcript production were underway 48-72 h after immunization. The absence of IL-4, IL-13, or B cells did not alter the early T cell proliferative response. The gamma1 and epsilon switch transcript production was still induced in the absence of IL-4, IL-13, or both, but at a reduced level, while the dominance of switching to IgG1 in the extrafollicular hapten-specific plasma cell response was retained. The up-regulation of IL-4 message was not reduced or delayed in the absence of B cells and was only marginally reduced by the absence of IL-13. It is concluded that signals delivered by dendritic cells, which are not dependent on the presence of IL-4, IL-13, or B cells, can prime virgin T cells and induce the early Th2 activities studied. These early events that direct virgin T cells toward Th2 differentiation contrast with the critical later role of Th2 cytokines in selectively expanding Th2 clones and driving further IL-4 synthesis.
The hematopathology subcommittee of the Mouse Models of Human Cancers Consortium recognized the need for a classification of murine hematopoietic neoplasms that would allow investigators to diagnose lesions as well-defined entities according to accepted criteria. Pathologists and investigators worked cooperatively to develop proposals for the classification of lymphoid and nonlymphoid hematopoietic neoplasms. It is proposed here that nonlymphoid hematopoietic neoplasms of mice be classified in 4 broad categories: nonlymphoid leukemias, nonlymphoid hematopoietic sarcomas, myeloid dysplasias, and myeloid proliferations (nonreactive). Criteria for diagnosis and subclassification of these lesions include peripheral blood findings, cytologic features of hematopoietic tissues, histopathology, immunophenotyping, genetic features, and clinical course. Differences between murine and human lesions are reflected in the terminology and methods used for classification. This classification will be of particular value to investigators seeking to develop, use, and communicate about mouse models of human hematopoietic neoplasms.
A consensus system for classification of mouse lymphoid neoplasms according to their histopathologic and genetic features has been an elusive target for investigators involved in understanding the pathogenesis of spontaneous cancers or modeling human hematopoietic diseases in mice. An international panel of scientists with expertise in mouse and human hematopathology joined with the hematopathology subcommittee of the Mouse Models for Human Cancers Consortium to develop criteria for definition and classification of these diseases together with a standardized nomenclature. The fundamental elements contributing to the scheme are clinical features, morphology, immunophenotype, and genetic characteristics. The resulting classification has numerous parallels to the World Health Organization classification of human lymphoid tumors while recognizing differences that may be species specific. The classification should facilitate communications about mouse models of human lymphoid diseases.
Exceptionally germinal center formation can be induced without T cell help by polysaccharide-based antigens, but these germinal centers involute by massive B cell apoptosis at the time centrocyte selection starts. This study investigates whether B cells in germinal centers induced by the T cell-independent antigen (4-hydroxy-3-nitrophenyl)acetyl (NP) conjugated to Ficoll undergo hypermutation in their immunoglobulin V region genes. Positive controls are provided by comparing germinal centers at the same stage of development in carrier-primed mice immunized with a T cell-dependent antigen: NP protein conjugate. False positive results from background germinal centers and false negatives from non-B cells in germinal centers were avoided by transferring B cells with a transgenic B cell receptor into congenic controls not carrying the transgene. By 4 d after immunization, hypermutation was well advanced in the T cell-dependent germinal centers. By contrast, the mutation rate for T cell-independent germinal centers was low, but significantly higher than in NP-specific B cells from nonimmunized transgenic mice. Interestingly, a similar rate of mutation was seen in extrafollicular plasma cells at this stage. It is concluded that efficient activation of hypermutation depends on interaction with T cells, but some hypermutation may be induced without such signals, even outside germinal centers.
B cell terminal differentiation is associated with the onset of high-level antibody secretion and cell cycle arrest. Here the cyclin-dependent kinase (CDK) inhibitor p18(INK4c) is shown to be required within B cells for both terminating cell proliferation and differentiation of functional plasma cells. In its absence, B cells hyperproliferate in germinal centers and extrafollicular foci in response to T-dependent antigens but serum antibody titers are severely reduced, despite unimpaired germinal center formation, class switch recombination, variable region-directed hypermutation, and differentiation to antibody-containing plasmacytoid cells. The novel link between cell cycle control and plasma cell differentiation may, at least in part, relate to p18(INK4c) inhibition of CDK6. Cell cycle arrest mediated by p18(INK4C) is therefore requisite for the generation of functional plasma cells.