T cells are central components of the adaptive immune system, recognizing and eliminating infected or abnormal cells. When these cells encounter a threat, they undergo activation, which prepares them to mount an immune response. T cell activation markers are specific molecules that appear on the surface of T cells, or are expressed internally, when they are stimulated. These markers serve as indicators of an active immune response, providing valuable insights in both research and clinical settings.
The T Cell Activation Pathway
T cell activation relies on a “two-signal” model. The first signal provides specificity when the T cell receptor (TCR) on the T cell surface recognizes and binds to a specific peptide antigen presented by a major histocompatibility complex (MHC) molecule on an antigen-presenting cell (APC). Helper T cells (CD4+ T cells) interact with peptide antigens on MHC class II molecules, found on professional APCs. Cytotoxic T cells (CD8+ T cells) recognize peptides on MHC class I molecules, found on nearly all nucleated cells.
The second signal provides co-stimulation, preventing inappropriate activation and ensuring a robust response. This involves the CD28 molecule on the T cell binding to B7 proteins on the APC. This co-stimulatory interaction, alongside TCR-MHC binding, triggers T cell activation, leading to proliferation and differentiation. Without this second signal, T cells may become anergic, a state of unresponsiveness.
Temporal Expression of Key Markers
Upon receiving both activation signals, T cells begin to express molecules on their surface. These temporal changes allow researchers and clinicians to track T cell activation. Early activation markers appear within hours of stimulation, indicating initial T cell engagement.
CD69 is an early activation marker, appearing on the T cell surface within hours after activation. Its expression is transient, peaking and then declining. CD69 retains activated T cells within lymphoid organs, preventing premature exit and promoting differentiation. Another early marker is CD25, the IL-2 receptor. Upregulation of CD25 on activated T cells increases their sensitivity to IL-2, a cytokine that promotes T cell proliferation and survival.
As T cell activation progresses, other markers become apparent, signifying sustained activation and the development of effector or memory functions. HLA-DR, an MHC class II molecule, is expressed on activated T cells, appearing within days and persisting longer than early markers. Its presence indicates a more advanced state of activation. CD44 is another marker upregulated on activated and memory T cells. While low levels are present on naive T cells, its sustained high expression distinguishes activated and antigen-experienced cells, reflecting their heightened migratory capacity.
Markers of T Cell Function and Exhaustion
Beyond simply indicating activation, specific markers reveal the functional state or specialized role of T cells during an immune response. These markers provide a more nuanced understanding of how T cells are behaving.
For assessing T cell proliferation, Ki-67 is a nuclear protein. Ki-67 is present during active phases of the cell cycle but absent in quiescent cells. Its detection indicates a T cell is actively dividing or has recently divided, making it a reliable indicator of T cell expansion.
Cytotoxic T lymphocytes (CTLs), which kill infected or cancerous cells, express intracellular molecules for their function. Granzyme B and Perforin are effector molecules stored within granules. Upon recognizing a target cell, perforin creates pores, allowing granzyme B to enter and induce programmed cell death (apoptosis). The presence of these markers signifies the cell’s capacity for direct cell-killing activity.
In chronic infection or cancer, T cells can enter a state of “exhaustion,” characterized by impaired function and reduced cytokine production. This state is marked by sustained expression of inhibitory receptors, also known as immune checkpoint molecules. Programmed cell death protein 1 (PD-1) and Cytotoxic T-Lymphocyte Antigen 4 (CTLA-4) are examples. While transiently upregulated during normal activation to regulate the immune response, their prolonged high expression is a hallmark of T cell exhaustion. Exhausted T cells with high PD-1 and CTLA-4 expression have diminished proliferative capacity and effector functions, representing a challenge in managing chronic diseases and cancer.
Laboratory Detection of Activation Markers
Identifying and quantifying T cell activation markers relies on techniques that analyze individual cells. These methods provide precise information about a sample’s immune status.
Flow cytometry is a technique used for detecting T cell activation markers. This technology involves suspending cells in a fluid stream and passing them through a laser beam.
Antibodies, proteins that bind to target molecules, are labeled with fluorescent dyes and added to the sample. When these antibodies bind to T cell markers, the laser excites the dyes, causing them to emit light. Detectors then measure the intensity and color of the emitted light, allowing quantification of marker expression on thousands of individual cells. This enables researchers to identify different T cell populations based on their unique marker profiles, such as activated CD4+ or CD8+ T cells.
Beyond flow cytometry, other methods offer spatial information about activated T cells within tissues. Immunohistochemistry (IHC) and immunofluorescence (IF) are techniques that use similar antibody-based detection to visualize markers in tissue sections. IHC uses an enzyme-based detection system that produces a colored precipitate, while IF uses fluorescent dyes. These methods allow localization of activated T cells within organs or tumors, providing context that complements cellular data from flow cytometry.