The Current Researches on T Follicular Helper Cells and Associated Diseases

T follicular helper (Tfh) cell is a new subpopulation of CD4 T cell family, whose differentiation is affected by Bcl-6, Blimp-1, STAT3, STAT5 and so on, and it could affect or decide the development of other subsets of CD4 T cells. The important function of Tfh cell is to help B cell mediate humoral immunity, many researches have proved that Tfh cells participate in the development of autoimmune disease, immunodeficient disease, tumor and infectious diseases.


Differentiation of Tfh cell
Tfh cells a r e generated from CD4 + T-cell and transcription repressor B-cell lymphoma 6 (Bcl-6) is one of the important transfer factor 1,2 and IL-21 is the key cytokine which causes differentiation and development of Tfh cell. 3Danelle 4 created the mice models deficient of IL-6 and IL-21 respectively, and found that only deficiency of IL-6 or IL-21 could not affect the development of Tfh cells, at the time of exiting both could be functional.IL-27 also affected Tfh cell because of enhancing the expression of IL-21. 5Fazilleau 6 took B10 and BR mice as the models of CI-EK limiting the response of Tfh cell, and proved that the combination ability of TCR with pMHCII and TCR signal intensity could effect Tfh cell differentiation.Moreover, the amount of Tfh cell would increase under the stimulation of adjuvant.Musculoaponeurotic fibrosarcoma (Maf) could induce CXCR5 expression, co-expression of Bcl-6 and Maf revealed that Bcl-6 and Maf cooperate in the induction of CXCR4, PD-1, and ICOS.These findings reveal that Bcl-6 and Maf collaborate to orchestrate a suite of genes that define core characteristics of human Tfh cell biology.

The Current Researches on T Follicular Helper Cells and Associated Diseases
Jianwei Zhou 1 , Cui Kong  The immunoglobulin-like glycoprotein CD226 is involved in the differentiation of naïve CD4 + T cells into effector cells.CD155 that is widely overexpressed on tumor cells, was identified as a counterreceptor of CD226 rendering many cancer cells sensitive to NK driven elimination.However, CD155 was also assigned to play role in the establishment of Tfh cells in the small intestine and the final maturation of CD8 positive thymocytes, the mice lacking CD226 are distinguished by virtually identical phenotypes as already reported for CD155 deficient mice: a paucity of follicular helper T cells in Peyer's patches and of terminally matured CD8 + T cells in thymus. 1380 is one of the few markers shared by human and murine memory B cells.There is an article which reported that CD80 -/-mice had fewer Tfh cells compared with that of controls, and residual Tfh cells failed to mature, with decreased ICOS and PD-1 expression and decreased synthesis of IL-21 mRNA.Mixed bone marrow chimeras demonstrated a B cellintrinsic requirement for CD80 expression for normal Tfh cell and plasma cell development. 14ditionally, IL-2 is a critical factor that regulates successful Tfh and B cell responses in vivo and regulates Tfh cell development. 11,15

Relationship between Tfh cell and other T-cell subgroups
As researches showed 16 , in the Tfh cells of mice, Bcl-6 could inhibit activity of GATA-3 and T-bet, both were inhibited when Bcl-6 was overexpressed, and the amount of Th1, Th2, Th17 decreased accordingly; while these subpopulation increased when Bcl-6 was insufficient.So Bcl-6 downregulated the differentiation and function of T cell subpopulation except for Tfh cells.Nakayamada 17 showed Tfh and Th1 cells share a transitional stage through the signal mediated by STAT4, which promotes both phenotypes, on the other hand, STAT4 could induce the transcription factor T-bet, while Tbet could repress Bcl-6 and other markers of Tfh cells, and constrained Tfh cell expansion and consequent GCs formation and antibody production, while T-bet could repress the functionalities of Tfh cell, and promote Th1 cell differentiation.
Another study reported that, during viral infection, most of the CD4 + T cells did not differentiate into Th1 cell but Tfh cell, this change will be beneficial for virus infection. 18Interestingly, Tfh cell affected other T cell subgroup not only by inhibiting their secretion of cytokines, but also by enhancing some cytokines which were secreted by other T subpopulations generally, to replace their secretory function. 19,20nterman 21 d e scribed a population of Foxp3 + Blimp-1 + CD4 + T cells constituting 10-25% of the CXCR5 high PD-1 high CD4 + T cells found in the GCs after immunization with protein antigens, which were named as follicular regulatory T (Tfr) cells, and shared phenotypic characteristics with Tfh cell and conventional Foxp3 + regulatory T (Treg) cells, but still they are distinct from both.Similar to Tfh cell, Tfr cell development depends on Bcl-6, signaling lymphocyte activation molecule associated protein (SLAM-associated protein [SAP]), CD28 and B cells; however, Tfr cells originate from thymusderived Foxp3 + precursors, not Tfh cells.Tfr cells are suppressive in vitro and limit Tfh cell and germinal center B cell numbers in vivo.In the absence of Tfr cells, an outgrowth of non-antigen-specific B cells in GC leads to fewer antigen-specific cells.Thus, the Tfh differentiation pathway is coopted by Treg cells (regulatory T cells) to control the germinal center response.
Besides, in the T cell families, Tfh cell is like Th17 cell, e.g., they all secrete IL-17, IL-21 and express ICOS, and their development depends on IL-6, IL-21, c-Maf and STAT3.However, Tfh cell does not secrete IL-17A, IL-17F and IL-22, the cytokines are actually secreted by Th17 cell, and its differentiation does not depend on RORγt (receptor related orphan receptor γ t) and TGF-β.

The mechanism of Tfh cell helping B cell
IL-21 not only improves the differentiation of Tfh cell, but also affects the function of Tfh cells on B cells.IL-21 could induce human B cells to produce large amount of IgM, IgG and IgA in some organs (tonsil, spleen or Peyer's patches) 22 , this is because IL-21R is clearly expressed on plasma cell from the human tonsil, the lymph node, and the spleen (secondary lymphoid organs, SLO); this effect was impaired when endogenous IL-21 production was blocked. 23ducible co-stimulator (ICOS) is the member of CD28 families, its ligand (ICOSL) mainly is expressed on the antigen presenting cells (APCs).Tfh cell secretes ICOS, B cells highly express ICOSL, their combination could induce expression of Bcl-6 24 ; thus, Tfh cells improve the quantity of IL-21, IL-4, IL-10, which enhance humoral immunity mediated by B cells. 25,26nterman 27 found a kind of cells, phenotype of which were CD4 + CD3 -, so called lymph tissue stimulating cells, could express OX40L and CD30L, which could combine with OX40 and CD30 on the surface of Tfh respectively, the combination promotes level of CXCR5 and co-location of Tfh cells in the GC.
There is a subset of Treg cells expressing CXCR5 and Bcl-6 that localize to the germinal centers in mice and humans.These CXCR5 + Bcl-6 + Treg cells are absent in the thymus, but can be generated de novo from CXCR5

Systemic lupus erythematosus
Nurieva 34 treated C57BL/6 mouse with N-ethyl-Nnitrosourea and successfully created Sanroque model with systemic lupus erythematosus (SLE).In the progress of SLE formation, roquein plays an important role.Roquein is a negative mediative protein for Tfh cell, when mutation happened, the function of inhibiting ICOS weakend, and large mount of Tfh cells accumulated in GCs, then the autoreaction mediated by B cells enhanced, resulting in autoimmune diseases, such as SLE. 35,361.2Rheumatoid arthritis Some scholars created mice models with collagen induce arthritis (CIA), and then they used monoantibody specific to ICOS to block ICOS signal pathway.As a result, the inflammatory symptom remitted and the production of abnormal GC in synovial membrane decreased.These showed that Tfh cell is the important element for the generation of RA. 37

Autoimmunological hepatitis
In the species of NTx-PD -/-mice models, there were lots of GCs, while Tfh existed in B regions, and could transfer into liver, high quantity of IL-21 secreted by them could activate CD8 + T cells which caused liver damage, resulting in autoimmunological hepatitis (AIH).However, inhibition of CCR6 and CCL20 of Tfh and CD8 + T cell could decrease the recurrence of AIH. 381.4Autoimmune thyroid disease Zhu 39 assessed circulating Tfh cells of sixty-five patients with autoimmune thyroid disease (AITD) by flow cytometry, analyzed the correlation between the percentages of CD4 + CXCR5 + ICOS high T cells and the levels of autoantibodies or hormones.Increased percentages of circulating Tfh cells in AITD patients were detected, and a positive correlation between the percentages of circulating Tfh cells and the serum concentrations of anti-TSH receptor-Ab/thyroperoxidase-Ab/thyroglobulin-Ab was confirmed. A  o sitive or modest relationship between the percentages of circulating Tfh cells and serum free T3 or free T4 was revealed in Grave's disease patients.After treatment, the percentage of circulating Tfh cells decreased in some Grave's disease patients.

Type 1 diabetes
Type 1 diabetes is an autoimmune disease caused by islet-reactive T cells which destroy insulin-producing β-cells.Diego 40 measured islet-specific CD4 + T cell regulation in T-cell receptor transgenic mice with elevated frequencies of CD4 + T cells recognizing hen egg lysozyme (HEL) autoantigen expressed in islet β-cells, and found that mouse anti-islet IgG antibodies formed as a consequence of excessive Tfh cell activity, progression to diabetes was ameliorated in the absence of B cells or when the B cells could not secrete islet-specific IgG, these indicated that Tfh cell play an important role in T1DM.

Other diseases
Conlon 41 created T cell-deficient mice models with monoclonal populations of TCR-transgenic CD4 + T cells, then transplanted heart from the same species, detected the differentiation of CD4 + T c e lls transferred to the recipients by follicular localization and by acquisition of signature phenotype.They found that the transferred CD4 + T cells differentiated into Tfh cells, and thought that Tfh cells take part in the rejection of organ transplantation.

Immunodefficiencies
According to in vivo or in vitro experiment of Xlinked lymphoproliferative disease (XLP), ICOS produced by CD4 + T cells decreased, there were something wrong with generation of the associated molecules which help for B cells, such as IL-4, IL-10, and the immunity of the patients declined.The same phenomenon existed in Hyper-IgM Syndrome (HIGM). 42,43

Infectious diseases
In the study of hepatitis B, researchers found that the frequency of Tfh cells in patients with immuneactive (IA) CHB was significantly higher than that of immune-tolerant (IT) CHB and healthy controls (HC), and the percentages of Tfh cell in IA patients were positively correlated with AST.Furthermore, the percentages of Tfh cells in CHB patients were significantly higher than that of HC.Treatment with adefovir dipivoxil reduced the frequency of Tfh cells, the concentrations of HBsAg and HBeAg, but increased the concentrations of HBsAb, HBeAb, IL-2 and IFN-γ in IA patients.Moreover, the frequency of splenic and liver Tfh cells in HBV-transgenic mice was higher than that of wild-type controls. 44 peripheral blood of HCV-infected patients, percentage of Tfh cells was higher than healthy controls, and statistically significant n e gative correlation was found between the percentage of Tfh cells and the HCV RNA load.These data suggested that Tfh cells may participate in HCV-related immune responses. 45In another study, the presence of intrahepatic Tfh cells in livers of HCV-infected patients was detected.The absolute number of liver Tfh cells progressively increased from nonresponders (NRs) to relapsers (RRs) to a maximum in sustained virological response (SVRs) patients, their amount is proportional t o t h e ultimate likelihood of SVR, with a progressive increase from NR to RR to SVR.Quantification of Tfh cells in the liver biopsy of these patients adds useful prognostic information. 46 the lymph node of rhesus macaques infected with SIVmac239, the progression of SIV infection was accompanied by increased numbers of well-delineated follicles containing GCs and Tfh cells, the density of PD-1 expression in lymph nodes progressively increased, and the rise in PD-1 + Tfh cells was followed by a substantial accumulation of Ki67 + B cells within GCs.Moreover, unlike in blood, major increases in the frequency of CD27 + memory B cells were observed in lymph nodes, indicating increased turnover of these cells which correlated with increases in total and SIV specific Ab levels. 47ere were reports that in the patients infected with worms, T cells in the GC produced lots of IL-4, phenotype of which was alike to Tfh cells, these data indicated that Tfh cell probably takes part in the quick reaction against worm infection. 27,48
Presently, Battistella 51 reported that, due to major Bcell infiltrate and CD10 positive in skin biopsy specimens, some patients were initially misdiagnosed as primary cutaneous follicle B-cell lymphoma, but rituximab-containing therapies were ineffective.In the further study, they found the biopsy specimens after treatment with rituximab showed medium to large-sized atypical T-cell skin infiltrate expressing Tfh cell markers (CD10, Bcl-6, PD-1, CXCL13 and ICOS).Finally, the diagnosis proposed for all patients was cutaneous Tfh lymphoma.According to the diagnosis, the patient with localized disease was successfully treated with radiotherapy.

Prospect
Conclusively, although the characteristic and function were identified, many questions remain unclear.What is the relation between Tfh cell and other T cell subpopulations?What are the detailed mechanisms of Tfh cell and helper B cell?Is there any interaction between Tfh cells and other immune cells?What are the functions of Tfh cells in the progression of lots of infectious diseases, autoimmune diseases and tumor?And is it possible for us to regulate Tfh cell that can influence the treatment of these diseases?There are many other questions also to be addressed.In near future, the researches on Tfh cells probably become a new focus to find out the possible mechanisms of immunodeficiency, infectious diseases, autoimmune diseases and tumor. 7 2 28oxp3 + natural Treg precursors.A lack of CXCR5 + Treg cells leads to greater germinal center reactions including germinal center B cells, affinity maturation of antibodies and the differentiation of plasma cells.These results unveil a Bcl-6-CXCR5 axis in Treg cells that drives the development of follicular regulatory Tfr cells that function to inhibit the germinal center reactions.28