Rebecca Newton

T Follicular helper cells express a distinctive transcriptional profile, reflecting their role as non-Th1/Th2 effector cells that provide help for B cells

Effector T cell responses have long been viewed in the context of the Th1/Th2 paradigm. Recently, a third major subset of nonpolarized effector T cells that provides help to B cells has been identified. These T cells, termed T follicular helper (TFH) cells, home to the B cell areas of secondary lymphoid tissue, through interactions mediated via the chemokine receptor CXCR5 and its ligand CXCL13. Affymetrix microarrays were used to identify transcription factors, cytokines, and cell surface molecules that underlie the differentiation pathways and functional properties of the TFH subset. The transcriptional profile of human CXCR5+ TFH cells was compared with that of Th1 and Th2 cells, which enabled the identification of numerous genes expressed preferentially by TFH cells, over the other effector subsets. Certain TFH genes were also expressed by B cells and thus appear to be particularly relevant for humoral immunity. Abs were used to confirm the expression of several factors. In particular, CD84 and CD200, the cytokine IL-21, and the transcription factor BCL6 were all strongly associated with TFH cells. Gene microarrays reveal a highly distinctive transcriptional profile for a third subset of effector T cells that differs markedly from Th1 and Th2 cells.

B Cell-Activating Factor Belonging to the TNF Family (BAFF)-R Is the Principal BAFF Receptor Facilitating BAFF Costimulation of Circulating T and B Cells

BAFF (B cell-activating factor belonging to the TNF family) is a cell survival and maturation factor for B cells, and overproduction of BAFF is associated with systemic autoimmune disease. BAFF binds to three receptors, BAFF-R, transmembrane activator and calcium modulator and cyclophilin ligand interactor (TACI), and B cell maturation Ag (BCMA). Using specific mAbs, BAFF-R was found to be the predominant BAFF receptor expressed on peripheral B cells, in both humans and mice, and antagonist mAbs to BAFF-R blocked BAFF-mediated costimulation of anti-μ responses. The other BAFF receptors showed a much more restricted expression pattern, suggestive of specialized roles. BCMA was expressed by germinal center B cells, while TACI was expressed predominantly by splenic transitional type 2 and marginal zone B cells, as well as activated B cells, but was notably absent from germinal center B cells. BAFF was also an effective costimulator for T cells, and this costimulation occurs entirely through BAFF-R. BAFF-R, but not TACI or BCMA, was expressed on activated/memory subsets of T cells, and T cells from BAFF-R mutant A/WySnJ mice failed to respond to BAFF costimulation. Thus, BAFF-R is important not only for splenic B cell maturation, but is the major mediator of BAFF-dependent costimulatory responses in peripheral B and T cells.

A novel leukocyte adhesion deficiency caused by expressed but nonfunctional β2 integrins Mac-1 and LFA-1

In the leukocyte adhesion deficiency (LAD)-1 syndrome, there is diminished expression of beta2(CD18) integrins. This is caused by lesions in the beta2-subunit gene and gives rise to recurrent bacterial infections, impaired pus formation, and poor wound healing. We describe a patient with clinical features compatible with a moderately severe phenotype of LAD-1 but who expresses the beta2 integrins lymphocyte function- associated molecule (LFA)-1 and Mac-1 at 40%-60% of normal levels. This level of expression should be adequate for normal integrin function, but both the patients Mac-1 on neutrophils and LFA-1 on T cells failed to bind ligands such as fibrinogen and intercellular adhesion molecule (ICAM)-1, respectively, or to display a beta2-integrin activation epitope after adhesion-inducing stimuli. Unexpectedly, divalent cation treatment induced the patients T cells to bind to ICAM-2 and ICAM-3. Sequencing of the patients two CD18 alleles revealed the mutations S138P and G273R. Both mutations are in the beta2-subunit conserved domain, with S138P a putative divalent cation coordinating residue in the metal ion-dependent adhesion site (MIDAS) motif. After K562 cell transfection with alpha subunits, the mutated S138P beta subunit was coexpressed but did not support function, whereas the G273R mutant was not expressed. In summary, the patient described here exhibits failure of the beta2 integrins to function despite adequate levels of cell-surface expression.

A fundamental bimodal role for neuropeptide Y1 receptor in the immune system

Psychological conditions, including stress, compromise immune defenses. Although this concept is not novel, the molecular mechanism behind it remains unclear. Neuropeptide Y (NPY) in the central nervous system is a major regulator of numerous physiological functions, including stress. Postganglionic sympathetic nerves innervating lymphoid organs release NPY, which together with other peptides activate five Y receptors (Y1, Y2, Y4, Y5, and y6). Using Y1-deficient (Y1−/−) mice, we showed that Y1−/− T cells are hyperresponsive to activation and trigger severe colitis after transfer into lymphopenic mice. Thus, signaling through Y1 receptor on T cells inhibits T cell activation and controls the magnitude of T cell responses. Paradoxically, Y1−/− mice were resistant to T helper type 1 (Th1) cell–mediated inflammatory responses and showed reduced levels of the Th1 cell–promoting cytokine interleukin 12 and reduced interferon γ production. This defect was due to functionally impaired antigen-presenting cells (APCs), and consequently, Y1−/− mice had reduced numbers of effector T cells. These results demonstrate a fundamental bimodal role for the Y1 receptor in the immune system, serving as a strong negative regulator on T cells as well as a key activator of APC function. Our findings uncover a sophisticated molecular mechanism regulating immune cell functions that can lead to stress-induced immunosuppression.

BAFF Augments Certain Th1-Associated Inflammatory Responses

B cell-activating factor belonging to the TNF family (BAFF; BLyS) is a critical regulator of B cell maturation and survival, and its overexpression in BAFF transgenic (Tg) mice results in the development of autoimmune disorders. BAFF also affects T cell function through binding to one of the BAFF receptors, BAFF-R. Using BAFF Tg mice, we examined a typical Th1-mediated response, the cutaneous delayed-type hypersensitivity reaction, and found a much greater degree of paw swelling and inflammation than in control mice. Importantly, delayed-type hypersensitivity scores correlated directly with BAFF levels in serum. Conversely, in a Th2-mediated model of allergic airway inflammation, BAFF Tg mice were largely protected and showed markedly reduced Ag-specific T cell proliferation and eosinophil infiltration associated with the airways. Thus, local and/or systemically distributed BAFF affects Th1 and Th2 responses and impacts on the course of some T cell-mediated inflammatory reactions. Our results are consistent with the idea that BAFF augments T cell as well as B cell responses, particularly Th1-type responses. Results in BAFF Tg mice may reflect the situation in certain autoimmune patients or virally infected individuals, because BAFF levels in blood are comparable.

Identification of T Cell-Restricted Genes, and Signatures for Different T Cell Responses, Using a Comprehensive Collection of Microarray Datasets

We used a comprehensive collection of Affymetrix microarray datasets to ascertain which genes or molecules distinguish the known major subsets of human T cells. Our strategy allowed us to identify the genes expressed in most T cell subsets: TCR αβ+ and γδ+, three effector subsets (Th1, Th2, and T follicular helper cells), T central memory, T effector memory, activated T cells, and others. Our genechip dataset also allowed for identification of genes preferentially or exclusively expressed by T cells, compared with numerous non-T cell leukocyte subsets profiled. Cross-comparisons between microarray datasets revealed important features of certain subsets. For instance, blood γδ T cells expressed no unique gene transcripts, but did differ from αβ T cells in numerous genes that were down-regulated. Hierarchical clustering of all the genes differentially expressed between T cell subsets enabled the identification of precise signatures. Moreover, the different T cell subsets could be distinguished at the level of gene expression by a smaller subset of predictor genes, most of which have not previously been associated directly with any of the individual subsets. T cell activation had the greatest influence on gene regulation, whereas central and effector memory T cells displayed surprisingly similar gene expression profiles. Knowledge of the patterns of gene expression that underlie fundamental T cell activities, such as activation, various effector functions, and immunological memory, provide the basis for a better understanding of T cells and their role in immune defense.

Human C5aR knock-in mice facilitate the production and assessment of anti-inflammatory monoclonal antibodies

Complement component C5a binds C5a receptor (C5aR) and facilitates leukocyte chemotaxis and release of inflammatory mediators. We used neutrophils from human C5aR knock-in mice, in which the mouse C5aR coding region was replaced with that of human C5aR, to immunize wild-type mice and to generate high-affinity antagonist monoclonal antibodies (mAbs) to human C5aR. These mAbs blocked neutrophil migration to C5a in vitro and, at low doses, both prevented and reversed inflammatory arthritis in the murine K/BxN model. Of ∼40 mAbs generated to C5aR, all potent inhibitors recognized a small region of the second extracellular loop that seems to be critical for regulation of receptor activity. Human C5aR knock-in mice not only facilitated production of high-affinity mAbs against an important human therapeutic target but were also useful in preclinical validation of the potency of these antagonists. This strategy should be applicable to other important mAb therapeutics.