Peripheral Helper T Cell
Overview
Peripheral helper T cells (Tph) are a CD4⁺ T cell subset defined by the phenotype CXCR5⁻PD-1⁺ within activated (CD38⁺HLA-DR⁺) CD4⁺ T cells. Unlike canonical circulating follicular helper T cells (cTfh, CXCR5⁺PD-1⁺), Tph cells lack the follicle-homing receptor CXCR5 and are proposed to provide B cell help in extrafollicular sites — including inflamed peripheral tissues rather than secondary lymphoid organ follicles. Tph cells were originally described in rheumatoid arthritis synovial tissue (Rao et al. 2017), where they drive local B cell differentiation and antibody production.
Ansari et al. (2025) demonstrate that Tph cells are the dominant activated CD4⁺ T cell subset in acute dengue — the first description of Tph dominance in an acute viral infection context.
Key Points from Literature
- ~75% of activated CD4⁺ T cells in acute dengue are Tph (CXCR5⁻PD-1⁺): In n=170 acute dengue patients, Tph greatly outnumber cTfh (CXCR5⁺PD-1⁺) among CD38⁺HLA-DR⁺ CD4⁺ T cells. This dominance persists across both primary and secondary infection (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue, prospective cohort n=170).
- Tph accumulate in severe dengue: Frequency of CXCR5⁻PD-1⁺ cells (of total CD4⁺) is significantly higher in severe dengue (WHO 2009 criteria) than in dengue with/without warning signs (p<0.05) (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).
- Tph produce IL-21 at levels comparable to cTfh: Intracellular cytokine staining and AIM assay demonstrate that Tph cells secrete IL-21 — the key EF differentiation cytokine identified in the Jenks2018 aNAV→DN2→plasmablast pathway. Tph cells also express CD40L, enabling cognate T-B interaction (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).
- scRNA-seq reveals helper vs. cytotoxic Tph subclusters: Analysis of 4,361 activated CD4⁺ T cells from 4 acute dengue patients identified: (1) an IL-21⁺ helper Tph subset expressing ICOS, MAF, TOX2; (2) a GZMB⁺ cytotoxic Tph subset expressing NKG7, KLRB1, HOPX. Only 13 TCR clonotypes were shared between the two subclusters, suggesting distinct clonal origins (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue, 10x Genomics scRNA-seq + scTCR-seq).
- Tph drive memory B cell → plasmablast differentiation: In T-B coculture experiments, sorted Tph cells drive class-switched memory B cells to differentiate into plasmablasts. Naive B cells respond poorly to Tph-provided help — contrasting with SLE, where naive cells are the dominant ASC precursors during flares (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue; cf. Tipton2015 - ASC Diversity and Origin in SLE).
- IL-21 is the dominant Tph effector cytokine for B cell help: Anti-IL-21 blocking antibody in T-B coculture reduces plasmablast output by ~60%, confirming IL-21 as the primary (but not exclusive) driver (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).
- Tph present in reactive lymph node tissue: CXCR5⁻PD-1⁺ T cells identified in lymph node samples (n=10), confirming tissue residency beyond peripheral blood (see Ansari2025 - Peripheral T Helper Subset Drives B Cell Response in Dengue).
Contradictions & Debates
- Whether Tph cells are a distinct lineage or a transient activation state of Tfh or effector T cells remains debated. The largely non-overlapping TCR clonotypes between helper and cytotoxic Tph in dengue argue against simple state plasticity, but the low number of shared clonotypes (13) could reflect limited sampling.
- In SLE, the dominant EF B cell precursor is the naive cell (aNAV/acN), driven by TLR7 signalling that is largely T cell-independent. In dengue, Tph cells preferentially drive memory B cell differentiation. This discrepancy raises the question of whether EF B cell activation in dengue operates through a fundamentally different (T cell-dependent) mechanism than in SLE (T cell-independent TLR7).
- The concurrent elevation of CXCL13 (a GC/Tfh biomarker) alongside Tph dominance challenges a purely extrafollicular model — it is possible that Tph cells are providing help at the T-B border (follicular/extrafollicular interface) rather than in purely extrafollicular sites.
Related Pages
PD-1, CXCR5, IL-21, CD40L, HOPX, TOX2, Extrafollicular Response, Germinal Center, Plasmablast, Memory B Cell, DN2 B Cell