It has been known for a while that T-helper (TH) cells are involved in the regulation of B-cell responses. The characterization of a precise subset of these specialized cells, called follicular T-helper cells (TFH), was first reported in 2000.1 TFH cells are characterized by their homing capacity in CXCL13-rich areas, such as B-cell follicles, through CXCR5 expression and their ability to support immunoglobulin production. The differentiation and homing of TFH, as well as their interactions with B cells, have been the subject of intense research and several markers have been identified as components of the TFH signature. Thanks to gene expression profiling carried out on peripheral T-cell lymphomas, a TFH signature has been reported in angioimmunoblastic T-cell lymphoma (AITL) and in follicular T-cell lymphoma.2–4 Several markers that identify TFH cells are now used in hematopathology to reinforce the diagnostic criteria of AITL. In this issue of the Journal, Marafioti et al.5 describe that inducible T-cell co-stimulatory molecule (ICOS), a CD28 homolog implicated in the regulation of T-cell differentiation and function, appears to be a relevant marker for the diagnosis of TFH-derived lymphoma such as AITL and follicular T-cell lymphoma. This review first summarizes the recent evidence on TFH cell differentiation and the present state of knowledge on molecules expressed by TFH cells. Then, from this perspective, the potential implications of TFH cells in lymphomagenesis and as part of the tumor microenvironment are covered.
Lineage and differentiation of follicular T-helper cells
TH cell responses are heterogeneous and effector function choice is imposed during initial activation of the naïve TH cells through the expression of transcription factors. It is now clear that TFH cells belong to a subset of TH cells that differ from the other TH cell subsets. Indeed, TH1 cells express STAT4, STAT1 and T-box transcription factor/T-bet, TH2 cells express GATA3, TH17 cells express retinoid related orphan receptor (RORγt) and regulatory T cells (Treg) express FOXP3. It has recently been shown that the TFH cell differentiation program is controlled by BCL6.6,7 TFH cells upregulate BCL6 which in turn blocks TH1, TH2 and TH17 cell differentiation by repressing their selective transcription factors.6 Moreover, BCL6 antagonizes the expression of Blimp-1 transcription factor, which is preferentially expressed by TH cells in the T zone area.6,8 BCL6 also represses the expression of miR-17-92, a micro RNA that down-regulates CXCR5 expression, which is essential for TFH cell function. Finally over-expression of BCL6 in activated TH cells induces expression of interleukin (IL)-6 receptor and IL-21 receptor, which are both required for TFH cell generation as shown by the role of IL-6 and IL-21 in TFH cell differentiation.9 ICOS is also implicated in the regulation of TFH cell differentiation. Once engaged with its ligand (ICOS-L), expressed on antigen-presenting cells including B cells, ICOS induces the production of helper cytokines such as IL-2, IL-4 and especially IL-10 and IL-21.10 ICOS deficiency is associated with a reduction of germinal center formation and fewer TFH cells, suggesting that ICOS signaling is essential in TFH cell generation.10 Recent reports described that TFH cells can produce significant amounts of IL-4, interferon-γ and IL-17 which are normally associated with TH2, TH1 and TH17 cells, respectively.1,11 Thus, TFH cells share phenotypic features with other TH cell lineages raising the question of whether TFH cells really do constitute a distinct lineage or whether they result from a conversion of effector TH cells that acquire TFH cell functions.
The development of follicular homing ability by activated TH cells is the first event in the process of TFH generation. Indeed, naïve T cells that express CD62L and CCR7 enter secondary lymphoid organs in the T-cell zones.8,11 Up-regulation of CXCR5, the receptor for CXCL13, a chemokine produced by follicular dendritic cells which promotes B-cell entry into the follicle, occurs after the interaction between antigen-presenting cells and naïve TH cells.8 Thanks to the expression of CXCR5 and down-regulation of CCR7, TFH cells move into the B follicle where they regulate antigen-specific B-cell responses via co-stimulatory signals delivered through CD28, OX40 and ICOS.11
Cell functions and markers of follicular T-helper cells
In addition to CXCR5, TFH cells express markers such as CD25, CD69, CD95, CD57 (in humans only), OX40 (CD134) and CD40L (CD154) and induce over-expression of activation-induced cytidine deaminase in B cells. TFH also produce cytokines such as IL-10 and IL-21 that promote B-cell survival and antibody production.10 An antigen-dependent T-B cell interaction is a critical initiating event in the development of effective B-cell immunity. This cognate interaction induces B cells to become short-lived plasma cells or to enter the germinal center where B cells will undergo maturation processes (immunoglobulin class-switching and somatic hypermutation) which will lead in the end to the development of high-affinity long-lived plasma cells and memory B cells.1,2,8 Mutations of CD40L or ICOS genes in mice or humans are associated with TFH cell deficiency and affect B-cell differentiation and humoral responses, indicating the important role of the co-stimulatory molecules in regulating B-cell responses. Of note, TFH cells express some molecules which play a key role in the stability of the antigen-dependent T cell-B cell interaction such as programmed cell death 1 (PD1), an immunoreceptor which favors strong cognate interactions with B cells in the germinal center, and the signaling lymphocytic activation molecule-associated protein (SAP) which is required to form stable contacts between B and T cells and supports TFH cell generation and germinal center formation.10,12 A recent study demonstrated that TFH can express high levels of B- and T-lymphocyte attenuator (BTLA), an inhibitory receptor of the CD28 superfamily which negatively regulates the immune response in synergy with the PD1/PDL1 inhibitory pathway.13
TFH cells seem to play an important role in systemic autoimmunity. Mice with systemic autoimmune diseases such as systemic lupus erythematosus show a constant increase of TFH cells.2 Furthermore, drugs that block ICOS or IL-21 signaling pathways improve autoimmune disorders such as diabetes, rheumatoid arthritis and murine models of systemic lupus erythematosus.11 TFH cells could, therefore, represent an interesting therapeutic target in autoimmune diseases.
Implications of follicular T-helper cells in lymphoma
AILT is an aggressive lymphoma characterized by a polymorphic infiltrate, marked proliferation of endothelial venules and a dense meshwork of follicular dendritic cells.14 Gene expression profiling studies have identified a specific signature corresponding to TFH cells.2–4 Furthermore, these studies have shown that TFH cells represent the putative cell of origin of AILT. TFH cells seem to be responsible for the clinico-biological aspects of AILT, including B-cell activation, hypergammaglobulinemia and proliferation of follicular dendritic cells. Typically the tumor cells of AILT have a TFH phenotype with expression of CD3, CD4, CXCR5, CD57 and BCL6. CD10, which is not expressed in normal TFH cells, but rather in a distinct subset of GC TH cells, is frequently present at the cell membrane of lymphoma cells.3,4 In addition, the neoplastic T cells frequently lose CD5 and CD7.14 Most interestingly, CXCL13, PD1 and SAP are also markers of malignant cells in AILT.15,16 This signature is not completely specific to AILT since it has also been reported in peripheral T-cell lymphoma not otherwise specified.15,17 Rare cases of peripheral T-cell lymphoma with a follicular growth pattern display a TFH phenotype and show some overlapping features with AILT, raising the question of a possible relationship between the two lymphoma types.5,18 In addition, in the study by Marafioti et al.,5 ICOS appeared to be expressed by tumor cells of both AILT and peripheral T-cell lymphoma not otherwise specified.
Nodular lymphocyte-predominant Hodgkin’s lymphoma is a monoclonal B-cell neoplasm characterized by nodular or nodular and diffuse proliferation of scattered large neoplastic B cells referred to as popcorn or lymphocyte-predominant cells.14 These cells interact with aggregates of follicular dendritic cells and immune cells such as small naïve B cells and T cells. Furthermore, nodules of this form of lymphoma are characterized by an increase of germinal center-derived CD57 T cells which are closely associated with neoplastic lymphocyte-predominant cells.19 These CD57 T cells also express markers reminiscent of germinal center TH cells such as PD1, BCL6, IRF4/MUM1 and c-MAF.19 Altogether these data suggest that some of these reactive cells could be TFH cells, but a more precise micro-dissection of the microenvironment is required to confirm this.
In follicular lymphoma, derived from germinal center B cells, tumor cells reside and proliferate in follicular structures in close association with TH cells and follicular dendritic cells.19 In order to proliferate, follicular lymphoma cells require cellular interactions such as those in normal germinal centers, including contacts with germinal center TH cells which express CD3, CD4, CD57, PD1 and CXCL13.19 Proliferation is stimulated by CD40-CD40L interaction, markers expressed on malignant B cells and TFH cells, respectively.19 This is further supported by the presence of significant amounts of PD1 in the germinal centers of patients with follicular lymphoma, as shown in a recent study.20 As for FOXP3 cells, high numbers of PD1 cells seem to have a favorable impact on progression in follicular lymphoma.20 Nevertheless, the exact nature of those PD1 cells remains unclear and further studies are needed to determine whether or not they correspond to TFH cells.
The increasing attention paid to TFH cells has revealed notable plasticity in the differentiation pathways of these cells. Beside canonical TH markers, the minimal TFH signature requires expression of CXCR5, PD1, BCL6, SAP and ICOS. These markers can also be individually expressed by other TH cell subsets. The use of immunohistochemistry to detect TFH cells on tissue sections is, therefore, questionable since only one or two markers can be labeled simultaneously. Hence, identification of TFH cells relies on both phenotype and function, which are closely related to the specific microenvironment in which TFH cells localize, requiring further studies based on multicolor cell imaging.
- Dr Laurent is an Assistant Professor of Pathology and works with Dr Brousset, Professor of Pathology in the University of Medicine in Toulouse.
- Dr Laurent (MD, PhD), Pr Brousset (MD, PhD) and Dr Fazilleau (PhD, Researcher) work in the Centre de Physiopathologie de Purpan, Inserm U563, in Toulouse.
- Related Original Article on page 356
- No potential conflicts of interests relevant to this article were reported.
- Schaerli P, Willimann K, Lang AB, Lipp M, Loetscher P, Moser B. CXC chemokine receptor 5 expression defines follicular homing T cells with B cell helper function. J Exp Med. 2000; 192(11):1553-62. Google Scholar
- Vinuesa CG, Tangye SG, Moser B, Mackay CR. Follicular B helper T cells in antibody responses and autoimmunity. Nat Rev Immunol. 2005; 5(11):853-65. Google Scholar
- de Leval L, Rickman DS, Thielen C, Reynies A, Huang YL, Delsol G. The gene expression profile of nodal peripheral T-cell lymphoma demonstrates a molecular link between angioimmunoblastic T-cell lymphoma (AITL) and follicular helper T (TFH) cells. Blood. 2007; 109(11):4952-63. Google Scholar
- Piccaluga PP, Agostinelli C, Califano A, Carbone A, Fantoni L, Ferrari S. Gene expression analysis of angioimmunoblastic lymphoma indicates derivation from T follicular helper cells and vascular endothelial growth factor deregulation. Cancer Res. 2007; 67(22):10703-10. Google Scholar
- Marafioti T, Paterson JC, Ballabio E, Chott A, Natkunam Y, Rodriguez-Justo M. The inducible T-cell co-stimulator (ICOS) molecule is expressed on subsets of T-cells and is a new marker of lymphomas of T follicular helper cell derivation. Haematologica. 2010; 95(3):432-9. Google Scholar
- Johnston RJ, Poholek AC, DiToro D, Yusuf I, Eto D, Barnett B. Bcl6 and Blimp-1 are reciprocal and antagonistic regulators of T follicular helper cell differentiation. Science. 2009; 325(5943):1006-10. Google Scholar
- Nurieva RI, Chung Y, Hwang D, Yang XO, Kang HS, Ma L. Generation of T follicular helper cells is mediated by interleukin-21 but independent of T helper 1, 2, or 17 cell lineages. Immunity. 2008; 29(1):138-49. Google Scholar
- Fazilleau N, Mark L, McHeyzer-Williams LJ, McHeyzer-Williams MG. Follicular helper T cells: lineage and location. Immunity. 2009; 30(3):324-35. Google Scholar
- Awasthi A, Kuchroo VK. Immunology. The yin and yang of follicular helper T cells. Science. 2009; 325(5943):953-5. Google Scholar
- King C, Tangye SG, Mackay CR. T follicular helper (TFH) cells in normal and dysregulated immune responses. Annu Rev Immunol. 2008; 26:741-66. Google Scholar
- Yu D, Batten M, Mackay CR, King C. Lineage specification and heterogeneity of T follicular helper cells. Curr Opin Immunol. 2009; 21(6):619-25. Google Scholar
- Qi H, Cannons JL, Klauschen F, Schwartzberg PL, Germain RN. SAP-controlled T-B cell interactions underlie germinal centre formation. Nature. 2008; 455(7214):764-9. Google Scholar
- M’Hidi H, Thibult ML, Chetaille B, Rey F, Bouadallah R, Nicollas R. High expression of the inhibitory receptor BTLA in T-follicular helper cells and in B-cell small lymphocytic lymphoma/chronic lymphocytic leukemia. Am J Clin Pathol. 2009; 132(4):589-96. Google Scholar
- WHO Classification of tumour of Haematopoetic and Lymphoid Tissues. IARC: Lyon; 2008. Google Scholar
- Roncador G, Garcia Verdes-Montenegro JF, Tedoldi S, Paterson JC, Klapper W, Ballabio E. Expression of two markers of germinal center T cells (SAP and PD-1) in angioimmunoblastic T-cell lymphoma. Haematologica. 2007; 92(8):1059-66. Google Scholar
- Yu H, Shahsafaei A, Dorfman DM. Germinal-center T-helper-cell markers PD-1 and CXCL13 are both expressed by neoplastic cells in angioimmunoblastic T-cell lymphoma. Am J Clin Pathol. 2009; 131(1):33-41. Google Scholar
- Rodriguez-Pinilla SM, Atienza L, Murillo C, Perez-Rodriguez A, Montes-Moreno S, Roncador G. Peripheral T-cell lymphoma with follicular T-cell markers. Am J Surg Pathol. 2008; 32(12):1787-99. Google Scholar
- Huang Y, Moreau A, Dupuis J, Streubel B, Petit B, Le Gouill S. Peripheral T-cell lymphomas with a follicular growth pattern are derived from follicular helper T cells (TFH) and may show overlapping features with angioimmunoblastic T-cell lymphomas. Am J Surg Pathol. 2009; 33(5):682-90. Google Scholar
- Carbone A, Gloghini A, Cabras A, Elia G. The germinal centre-derived lymphomas seen through their cellular microenvironment. Br J Haematol. 2009; 145(4):468-80. Google Scholar
- Carreras J, Lopez-Guillermo A, Roncador G, Villamor N, Colomo L, Martinez A. High numbers of tumor-infiltrating programmed cell death 1-positive regulatory lymphocytes are associated with improved overall survival in follicular lymphoma. J Clin Oncol. 2009; 27(9):1470-6. Google Scholar