there has been a rapidly evolving literature around a unique CD4+ T‐cell subset that takes part in the natural functions of normal germinal centers. These cells, called T follicular helper (Tfh) cells, support B cells in the context of the germinal center reaction. They reveal a distinctive phenotype with expression of the germinal center markers BCL6 and CD10 together with CD57, ICOS and CD279/PD1 and produce the chemokine CXCL13 as well as its receptor CXCR5. CXCL13 causes proliferation of follicular dendritic cells and facilitates the migration of B and T lymphocytes expressing CXCR5 into the germinal center.
The T‐cell System as a Frame for Peripheral T‐cell Lymphoma: Taking Plasticity into Account
The mature T‐cell system consists of different subsets of lymphoid T cells variably trafficking between peripheral blood, lymphoid, and peripheral tissues. These cells display diversified functional phenotypes, participating in immune responses as effectors or though the orchestration of other immune cellular and non‐cellular contributors.
This gross distinction also implies that one major branch of the T‐cell functional differentiation tree displays transcriptional and synthetic machineries allowing cytotoxic effector functions, including for example EOMES transcription factor expression, granzymes, perforins, and key proinflammatory cytokines such as TNFα and interferon gamma (IFN‐γ). Another major branch is strictly bound, in its activity, to microenvironmental clues of the immune contexture.
In this regard, the described differentiated phenotypes of CD4+ Th‐cell subsets are known to include Th1 and Th2 clusters identified by the activity of T‐bet and GATA3 transcription factors [2]; the Th17 and Th22 clusters driven by RORC1 transcriptional regulation [3, 4]; the FOXO1 and interferon regulatory factor 4 (IRF4) controlled Th9 cluster [5]; the Tfh cluster, which is dependent upon BCL6 and TOX2 activity [6], and the regulatory T‐cell cluster associated with FOXP3 [7]. This repertoire of discrete and differentiated cells imparts a considerable degree of plasticity to the immune system, enabling it to respond potently with high selectivity.
Th subsets of CD4 positive cells are defined by the activity of selected transcription factors, including diversified signal transducers and activators of transcription (STAT) family members, whose signaling and preferential synthesis of specific cytokines, may respond to polarizing stimuli from the surrounding environment by reshaping their phenotypic differentiation, eventually undergoing conversion to a different functional subset [8, 9]. Specific cytokines that characterize most of the pathogen‐associated or sterile inflammatory responses, such as IL6, TNFα, and IL12 may drive Th cell polarization toward Th1 (STAT4‐dominant), Tfh, Th17, or Th22 fates (all three STAT3‐dominant), according to their relative abundance and association with other regulatory cytokines such as IL1b, IL21, and IL23 [8]. Similarly, the pleiotropic cytokine transforming growth factor beta is involved in the induction and/or conversion of Treg (STAT5‐dominant), Th2 and Th9 (both STAT6‐dominant) in combination with other polarizing cytokines such as IL2 and IL4 [8].
With this background, the predominant functional polarization of T cells within a specific immune context is fated to change according to the timing of the response and to the establishment of different immune/stroma interfaces. In this setting, a notable example is represented by the induction of tertiary lymphoid structures in tissues of patients with autoimmune diseases or cancer. In this context, the polarization of programmed cell death 1 receptor‐positive (PD1+) Tfh cells from adaptive or tissue‐resident innate‐like T cells [10] is entwined with the formation of a follicular dendritic cell network from vessel‐associated mesenchymal stromal cells (Figure 1.2) [11]. This process leads to the establishment of a specialized stroma‐T‐cell interface that may be either aberrantly expanded or variably disrupted in different lymphoma subtypes related to the germinal center, such as is seen with angioimmunoblastic T‐cell lymphoma [12], and the follicular lymphomas [13], respectively.
Besides primary polarization and conversion dictated by the cytokine milieu and stromal micro niches, an emerging role for TCR specificity in determining fate specification of tissue‐resident T cells at sites of persistent antigenic challenge, such as the intestinal lamina propria, is emerging [14]. Clonal TCR rearrangements of peripheral FOXP3+ Treg (pTreg) promote the generation of distinct pTreg CD4 intraepithelial T‐cell phenotypes, which show comparable dependence on microbiota‐derived antigenic stimuli and different reliance on intraclonal competition. Intraclonal competition driven by TCR‐antigen signaling is a limiting factor in natural and pTreg development, with low clonal precursor frequencies being required for their generation [15].
The specific issue of intraclonal competition and “small niche” requirement as a potential determinant of T‐cell subclones’ fate specification in the presence of self‐ and/or microbiota‐derived antigenic represents a still unchallenged level of complexity in peripheral T‐cell lymphoma (PTCL) research. In this regard, single‐cell RNA sequencing (Sc‐RNA‐seq) provides a dramatically higher level of resolution of the developmental and functional heterogeneity of T‐cell subsets, which is applicable to the PTCL setting.
Sc‐RNA‐seq has provided reference maps for circulating and tissue‐based T cells and their functional hierarchies [16]. Among relevant transcriptional differences between peripheral blood and tissue‐based T cells, cytoskeleton, cell–matrix interaction, and cell proliferation modules have emerged, underscoring the transcriptional imprint of tissue microenvironment. This information must be considered in the interpretation of gene expression profiling studies on PTCL, which have mostly relied on the differential comparison with normal T‐cell subsets from peripheral blood and/or cell lines.
Figure 1.2 Representative immunohistochemical staining for PD1 (upper left) and NGFR/CD271 (upper right) highlighting the presence of PD1+ T follicular helper cells and of a formed meshwork of follicular dendritic cells within a tertiary lymphoid follicle in a case of non‐small‐cell lung carcinoma. Immunohistochemical staining for CD146 (lower left) and CD23 (lower right) highlighting the early branching of perivascular CD146+ mesenchymal elements that partially display CD23 expression as a marker of follicular dendritic cell differentiation in the same non‐small‐cell lung carcinoma.
Source: Claudio Tripodo, Stefano Pilleri.
Moreover, at the Sc‐RNA‐seq level, a clear dichotomy in the CD8+ effector T‐cell cluster emerges as conserved across tissue sites, based on the predominance of cytotoxicity‐related genes or cytokine and chemokine genes [16].
Tissue‐conserved Sc‐RNA‐seq transcriptional clustering also identifies distinct Treg, CD4+ naïve/central memory resting and CD4+/CD8+ resting clusters, IFN response, and proliferation gene clusters characterizing different CD4+ T‐cell activation states [16]. Interestingly, hallmarks of such transcriptional modules feature the concomitant expression of genes coding for transcription factors involved in divergent‐fate specifications (e.g. FOXP3, IRF4, and TOX2), highlighting the dynamical regulation of promiscuous transcription factors [17, 18]. Again, this concept may represent a relevant note of caution in the interpretation of transcription factor expression data in PTCL clonal populations as hallmarks of stable clone differentiative/phenotypic states.
The expression and activity of specific transcription factors involved in fate specification, such as T‐bet and IRF4, is bound to that of the pleiotropic transcription factor MYC in enabling TCR‐driven immune function through the regulation of T‐cell proteomic and metabolic landscape [19]. MYC proficiency has been demonstrated to be required for adapting protein expression to TCR engagement in CD4+ and CD8+ cells. Through the regulation of amino acid transporter expression, MYC impacts on translational activity and on the associated increase of effector T‐cell biomass (Figure
