CD4 T Cell Markers: Insights into Subset Functions

CD4 T cells are central regulators of the immune response, orchestrating defense mechanisms against pathogens while maintaining tolerance to self-antigens. Their functional diversity is reflected in distinct subsets that contribute to immunity in specialized ways. Understanding the markers associated with these subsets provides valuable insights into their roles in health and disease.

To explore how CD4 T cell subsets function, it is essential to examine key surface markers, transcription factors, cytokine profiles, and distinctions between memory and effector states.

Cell Surface Markers for Helper T Cells

Helper T cells, defined by their expression of CD4, rely on surface markers that dictate their interactions with other immune cells and influence their specialization. CD4 itself enhances T cell receptor (TCR) signaling by stabilizing interactions with major histocompatibility complex class II (MHC-II) molecules on antigen-presenting cells. CD3, a component of the TCR complex, is universally present on all T cells and plays a role in signal transduction.

CD25, the alpha chain of the interleukin-2 (IL-2) receptor, is highly expressed on regulatory T cells (Tregs) and transiently upregulated on activated conventional CD4 T cells. It facilitates responsiveness to IL-2, a cytokine essential for T cell proliferation and survival. CXCR5 is predominantly found on follicular helper T cells (Tfh), enabling their migration into B cell follicles, where they support antibody production. CCR6 is associated with Th17 cells, guiding their movement to mucosal sites involved in host defense and inflammation.

PD-1, a co-inhibitory receptor, modulates activation thresholds, preventing excessive immune responses that could lead to tissue damage. Conversely, CD40L (CD154) is a co-stimulatory molecule that enhances interactions with antigen-presenting cells, particularly dendritic cells and B cells, promoting immune activation. The balance between stimulatory and inhibitory signals from these surface markers determines the functional output of helper T cells in different physiological contexts.

Transcription Factors in Subset Differentiation

The functional specialization of CD4 T cells is orchestrated by lineage-defining transcription factors that regulate gene expression and guide differentiation into distinct subsets. These transcription factors act as molecular switches, integrating signals from the extracellular environment to establish and maintain subset identity.

T-bet, encoded by the TBX21 gene, is the master regulator of Th1 differentiation. Its expression is induced by interferon-gamma (IFN-γ) and interleukin-12 (IL-12), leading to the activation of genes associated with Th1 function. T-bet promotes IFN-γ transcription while repressing GATA3, the key regulator of Th2 differentiation. By modifying chromatin accessibility, T-bet ensures stable commitment to the Th1 lineage.

GATA3 directs Th2 differentiation, promoting the expression of interleukin-4 (IL-4), interleukin-5 (IL-5), and interleukin-13 (IL-13). These cytokines establish Th2 functionality, particularly in allergic reactions and helminth immunity. GATA3 binds to regulatory regions of Th2 cytokine genes while inhibiting T-bet-mediated Th1 differentiation, illustrating the competitive nature of subset specification.

Th17 differentiation is governed by RORγt, encoded by the RORC gene. Induced by transforming growth factor-beta (TGF-β) in combination with interleukin-6 (IL-6) and interleukin-21 (IL-21), RORγt activates interleukin-17A (IL-17A) and interleukin-17F (IL-17F). It binds to enhancer elements of Th17-associated genes while repressing pathways leading to Th1 or Th2 differentiation. STAT3 cooperates with RORγt to reinforce Th17 lineage stability.

T follicular helper (Tfh) cells require Bcl-6 for their development. Bcl-6 represses T-bet, GATA3, and RORγt, ensuring a distinct Tfh identity. It enables the expression of CXCR5, which facilitates localization to germinal centers, and modulates co-stimulatory molecules such as ICOS and PD-1, which are critical for interactions with B cells.

Regulatory T cells (Tregs) are defined by Foxp3, a transcriptional repressor that silences effector T cell programs while activating suppressive functions. Foxp3 expression is stabilized by epigenetic modifications, ensuring Treg identity even in inflammatory environments. Foxp3 also interacts with STAT5, which is activated in response to IL-2 signaling, further reinforcing the Treg phenotype.

Cytokine Profiles Shaping Functions

The functional identity of CD4 T cell subsets is dictated by their cytokine profiles, which influence interactions with surrounding cells and shape immune responses.

Th1 cells produce IFN-γ, which amplifies their differentiation and directs macrophage activity. IFN-γ enhances antigen presentation and promotes reactive oxygen species production, aiding pathogen clearance. It also suppresses competing CD4 T cell fates by downregulating factors associated with Th2 and Th17 differentiation.

Th2 cells secrete IL-4, IL-5, and IL-13, orchestrating responses related to tissue repair and barrier defense. IL-4 promotes Th2 differentiation and immunoglobulin class switching in B cells, favoring IgE production. IL-5 recruits eosinophils, while IL-13 contributes to mucus production and smooth muscle contraction. These cytokines are implicated in allergic asthma, where excessive Th2 activity leads to airway hyperresponsiveness.

Th17 cells secrete IL-17A, IL-17F, and IL-22, which regulate epithelial integrity and inflammatory responses. IL-17A and IL-17F stimulate fibroblasts and epithelial cells to produce chemokines that attract neutrophils, reinforcing barrier protection. IL-22 promotes epithelial cell proliferation and antimicrobial peptide production, supporting tissue repair. Excessive IL-17 signaling has been linked to autoimmune conditions such as psoriasis and inflammatory bowel disease.

Tfh cells rely on IL-21, which enhances B cell differentiation and antibody production. IL-21 amplifies germinal center reactions by promoting plasma cell formation and affinity maturation. It also modulates Tfh survival and function, sustaining long-term interactions with B cells. Dysregulated IL-21 signaling has been associated with autoantibody production in systemic lupus erythematosus.

Markers of Memory vs Effector States

CD4 T cells transition between effector and memory states based on antigen exposure and activation history. Effector CD4 T cells, which arise during acute immune responses, express CD69 and CD25, markers of recent activation. CD69 limits tissue egress, while CD25 enhances IL-2 sensitivity, supporting rapid proliferation. These cells also upregulate CD44, a glycoprotein involved in adhesion and migration, facilitating movement to inflamed tissues. Most effector cells are short-lived, with only a subset persisting as memory cells.

Memory CD4 T cells provide long-term protection and are classified by CCR7 and CD62L expression. Central memory T cells (T_CM) retain CCR7 and CD62L, homing to lymphoid tissues for rapid secondary responses. Effector memory T cells (T_EM) lack CCR7 but express CD95 (Fas), allowing circulation in peripheral tissues for immediate reactivation. Tissue-resident memory T cells (T_RM) remain in non-lymphoid tissues through CD103 and CD69 expression, providing localized immunity.