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T lymphocytes (T cells) represent one of the major cell populations of the immune system. Named by the place of their development, the thymus, several types can be distinguished as the αβ T cells, the γδ T cells, the mucosa-associated invariant T cells (MAIT), and the natural killer T (NKT) cells. The αβ lineages of CD4+ THelper and the CD8+ T cytotoxic cells with the T cell receptor (TCR) composed of α- and β-chain are major players of the adaptive immune system. In the thymus, CD4+ and CD8+ single positive (SP) αβ cells represent the ultimate result of positive and negative selection of CD4+CD8+ double positive (DP) thymocytes. The DP population derives from the double negative (DN) thymocytes that develop from bone marrow-derived progenitors through different stages (DN1-DN4) that are characterized by CD25 and CD44 surface expression.
NFATc1, a member of the Nuclear Factor of Activated T cells (NFAT) transcription factors family, is critically involved in the differentiation and function of T cells. During thymocyte development, the nuclear expression of NFATc1 reaches the highest level at the DN3 (CD44-CD25+) stage. The hematopoietic cell-specific ablation of NFATc1 activity results in an arrest of thymocyte differentiation at the DN1 (CD44+CD25-) stage. On the other hand, over-expression of a constitutively active version of NFATc1 results in an impaired transition of DN3 cells to the DN4 (CD44-CD25-) stage, suggesting that a certain threshold level of NFATc1 activity is critical at this point.
ChIP-seq and RNA-seq analysis allowed us the identification of NFATc1/A target genes involved in lineage development as the Tcra and Tcrb gene loci. Furthermore, we identified multiple NFATc1-regulated genes that are involved in γδ T cell development. In the mouse models, Rag1Cre-Nfatc1fl/fl and Rag1Cre-E2fl/fl, in which the activity of NFATc1 or inducible NFATc1 in the latter is impaired during the early stages of thymocyte development, we observed increased numbers of γδ T cells. These γδ T cells showed an unusual overexpression of CD4, a lack of CD24 expression, and overexpression of the anti-apoptotic gene Bcl2a1a.
We hypothesize that during the DN stages NFATc1 plays an important role in regulating crucial steps of αβ thymocyte development and when NFATc1 activity is missing this may disturb αβ development resulting in alternative cell fates like γδ T cells.
Burkitt lymphoma (BL) is a highly aggressive B cell malignancy. Rituximab, a humanized antibody against CD20, in a combination with chemotherapy is a current treatment of choice for B-cell lymphomas including BL. However, certain group of BL patients are resistant to Rituximab therapy. Therefore, alternative treatments targeting survival pathways of BL are needed.
In BL deregulation of MYC expression, together with additional mutations, inhibits differentiation of germinal centre (GC) B cells and drives proliferation of tumor cells. Pro-apoptotic properties of MYC are counteracted through the B-cell receptor (BCR) and phosphoinositide-3-kinase (PI3K) pathway to ensure survival of BL cells. In normal B-cells BCR triggering activates both NF-κB and NFAT-dependent survival signals. Since BL cells do not exhibit constitutive NF-κB activity, we hypothesized that anti-apoptotic NFATc1A isoform might provide a major survival signal for BL cells.
We show that NFATc1 is constitutively expressed in nuclei of BL, in BL cell lines and in Eµ-Myc–induced B cell lymphoma (BCL) cells. Nuclear residence of NFATc1 in these entities depends on intracellular Ca2+ levels but is largely insensitive to cyclosporine A (CsA) treatment and therefore independent from calcineurine (CN) activity. The protein/protein interaction between the regulatory domain of NFATc1 and DNA binding domain of BCL6 likely contributes to sustained nuclear residence of NFATc1 and to the regulation of proposed NFATc1-MYC-BCL6-PRDM1 network in B-cell lymphomas.
Our data revealed lack of strict correlation between the expression of six NFATc1 isoforms in different BL-related entities suggesting that both NFATc1/alphaA and -betaA isoforms provide survival functions and that NFATc1alpha/betaB and -alpha/betaC isoforms either do not possess pro-apoptotic properties in BL cells or these properties are counterbalanced. In addition, we show that in BL entities expression of NFATc1 protein is largely regulated at post-transcriptional level, including MYC dependent increase of protein stability.
Functionally we show that conditional inactivation of Nfatc1 gene in Eµ-Myc mice prevents development of BCL tumors with mature B cell immunophenotype (IgD+). Loss of NFATc1 expression in BCL cells ex vivo results in apoptosis of tumor cells.
Together our results identify NFATc1 as an important survival factor in BL cells and, hence, as a promising target for alternative therapeutic strategies for BL.
In this work we wanted to investigate the role of NFATc1 in lymphocyte physiology and in pathological conditions (eg. psoriasis). NFATc1 is part of the signal transduction
pathways that regulates B cells activation and function. NFATc1 has different isoforms that are due to different promoters (P1 and P2), polyadenylation and alternative splicing. Moreover, we tried to elucidate the points of interactions between the NFAT and the NF-κB pathways in
activated B-cell fate. NFAT and NF-κB factors share several properties, such as a similar mode of induction and architecture in their DNA binding domain. We used mice which over-express a constitutive active version of NFATc1/α in their B cells with -or without- an ablated IRF4. IRF4 inhibits cell cycle progression of germinal center B cell-derived Burkitt’s lymphoma cells and
induces terminal differentiation toward plasma cells. Our experiments showed that a ‘double hit’ in factors affecting B cell activation (NFATc1 in this case) and late B cell Differentiation (IRF4 in this case) alter the development of the B cells, lead to increase in their numbers and increase in stimulation induced proliferation. Therefore, the overall picture indicates a link between these 2 genes and probable carcinogenic alterations that may occur in B cells.
We also show that in splenic B cells, c-Rel (of the NF-κB canonical pathway) Support the induction of NFATc1/αA through BCR signals. We also found evidence that the lack of NFATc1 affects the expression of Rel-B (of the NF-κB non-canonical pathway). These data suggest a tight interplay between NFATc1 and NF-κB in B cells, influencing the competence of B cells and their functions in peripheral tissues.
We also used IMQ-induced psoriasis-like inflammation on mice which either lack NFATc1 from B cell. Psoriasis is a systemic chronic immunological disease characterized
primarily by abnormal accelerated proliferation of the skin keratinocytes. In psoriasis, the precipitating event leads to immune cell activation. Our experiments showed that NFATc1 is needed for the development of psoriasis. It also showed that IL-10 is the link that enables NFAT
from altering the B cell compartment (eg Bregs) in order to affect inflammation. The important role of B cell in psoriasis is supported by the flared up psoriasis-like inflammation in mice that lack B cells. Bregs is a special type of B cells that regulate other B cells and T cells; tuning the immunological response through immunomodulatory cytokines.