CD4+ T cells, sometimes called “helper” T cells, play a more central role in the immune system than their name may imply. Once activated, CD4+ T cells release chemical messengers called cytokines that help direct the immune response.
As part of the adaptive immune system, they can stimulate other immune cells, such as plasma cells, to produce antibodies, activate cytotoxic T cells to kill infected or cancerous cells, and enhance the activity of macrophages—all to orchestrate a coordinated attack on a specific antigen.
What are CD4+ T Cells?
CD4+T cells are a subset of T lymphocytes that express the CD4 coreceptor protein on their cell surface. Like other T cells, they originate in the thymus, generated from bone marrow progenitor stem cells that lack the coreceptors necessary to be considered mature T cells.
As T cells undergo thymopoiesis, they are sorted through positive and negative selection steps that test for and enable them to recognize and respond to the presentation of antigens. This process is when immature T cells gain functional receptors like CD8 and CD4.
Helper T cells are differentiated from other T cells based on the presence of CD4 on their membrane. CD4 is a glycoprotein that, like CD8, is a coreceptor for the T cell receptor (TCR). It is a member of the immunoglobulin superfamily and has four domains that are exposed on the extracellular side of the cell.
CD4, in tandem with TCR, interacts with major histocompatibility complex class II (MHC II) molecules on the surface of antigen-presenting cells (APCs). This allows helper T cells to recognize antigens.
CD4+ Helper T-Cell Function
Helper T cells help regulate immunity through two primary functions: cytokine-mediated and cell-to-cell contact activation of immune cells.
- Cytokine-mediated activation: CD4+ T cells generate and release small proteins called cytokines that act as chemical messengers between cells. Cytokines bind to specific receptors on other cells’ surfaces, activating or suppressing immune responses within those cells by triggering intracellular signaling cascade events. Cytokines released by helper T cells modulate the effector actions of cytotoxic T cells (CD8+), memory T cells, B cells, and natural killer (NK) cells. Some of the major cytokines secreted by CD4+ cells include interferon-gamma (IFN-γ), tumor necrosis factor-beta (TNF-β), IL-2, IL-4, IL-5, IL-6, IL-9, IL-10, and IL-13.
- Contact activation: CD4+ cells produce CD40 ligand (CD40L) on their surface, binding to the CD40 receptor found on B cells and CD8+ T cells to initiate certain contact activation events. The direct binding of CD40L by CD40 plays a role in plasma cell differentiation of B cells (along with cytokines released by helper T cells). CD40L/CD40 binding also enables CD8+ T cells to differentiate into memory cells. Direct CD4+ antigen presentation to CD8+ T cells likely also contributes to the memory acquisition process. In addition, CD4+ cells bind to macrophages and dendritic cells to upregulate the maturation and activation of these cell types.
CD4+ T Cell Differentiation and Activation
Activation of helper T cells occurs when their TCRs and CD4 coreceptors recognize and bind antigens presented by MHC II molecules on the surface of APCs. In addition to TCR signaling, CD4+ T cells require co-stimulatory signals to fully activate.
Co-stimulatory molecules, such as CD80 and CD86, reside on the surface of APCs and interact with the CD28 receptors on CD4+ T cells. Once antigen recognition and co-stimulation signals occur, helper T cells become activated, which triggers a series of intracellular signaling events that produce cytokines and other effector molecules.
Activated helper T cells then undergo clonal expansion, resulting in the proliferation of a large population of CD4+ cells. The type of cytokines present during T cell activation trigger specific signaling pathways that result in the different CD4+ subsets.
CD4+ T Cell Subsets
Classically, naive CD4+ T cells differentiate into T-helper 1 (TH1) and T-helper 2 (TH2) subsets.
- TH1 cells release cytokine molecules to activate macrophages, which specialize in engulfing and eliminating foreign cells and substances from the body through phagocytosis. TH1 cells also activate cytotoxic (CD8+) T cells. Interleukin-12 (IL-12) and interferon-gamma (IFN-γ) are crucial in driving CD4+ T cells toward the TH1 subset.
- TH2 cells release cytokines that activate B cells, which differentiate into plasma cells that create and release antibodies. The antibodies have a wide range of functions from identifying viruses and diseased cells to inducing an allergic response and prompting the body to cough and sneeze to get rid of foreign substances. Interleukin-4 (IL-4) and IL-2 are the key cytokines that drive CD4+ T cells toward the TH2 subset. IL-4 and IL-2 induce the expression of transcription factors like STAT5, STAT6, and GATA3, which further promotes TH2 differentiation
However, additional subsets continue to be identified and studied. T-helper 17 cells, follicular helper T cells, and T-helper 9 cells are a few of these newly discovered subsets. Each of these has a characteristic cytokine profile that couples with the activation of transcription factors and lineage-specific epigenetic modification to alter phenotype and function.
Although regulatory T cells (Treg) are considered distinctly differentiated cells when they leave the thymus, they share a high functional reliance on their CD4 coreceptors. They are often considered an additional subset of CD4+ cells.
It’s important to note that CD4+ T cell differentiation is a complex and dynamic process influenced by multiple factors. The combination of cytokines, duration of exposure, strength of signaling, and interaction with other immune cells and their products all contribute to the final differentiation fate of CD4+ T cells.
CD4+ T Cell Exhaustion
CD4+ T cell exhaustion refers to a state of functional impairment and loss of effector functions that occurs during chronic infections, prolonged antigen exposure, or persistent immune stimulation. Exhausted helper T cells progressively lose their ability to secrete cytokines and perform their CD4 functions effectively. This includes reduced production of key cytokines, such as interferon-gamma (IFN-γ), interleukin-2 (IL-2), and tumor necrosis factor-alpha (TNF-α).
Exhausted CD4+ T cells have reduced proliferative potential, leading to an inability to expand and generate a robust immune response. In addition, they often express high levels of inhibitory receptors, such as programmed cell death protein 1 (PD-1), cytotoxic T-lymphocyte-associated protein 4 (CTLA-4), T-cell immunoglobulin and mucin-domain containing-3 (TIM-3), and lymphocyte-activation gene 3 (LAG-3). Engagement of these receptors by their ligands on immune cells leads to the inhibition of cell activation and effector functions.
Exhausted CD4+ T cells also exhibit altered gene expression profiles and epigenetic modifications compared to functional CD4+ T cells. This transcriptional and epigenetic rewiring contributes to the loss of effector functions and the acquisition of exhaustion-associated phenotypes.
CD4+ T cell exhaustion is also the primary result of certain cancers and viruses, including human immunodeficiency virus 1 (HIV-1), which specifically infects and destroys these cells. The resulting increase in CD4+ cell exhaustion and decline in cell count weakens the immune system, which can ultimately result in acquired immunodeficiency syndrome (AIDS), a condition characterized by increased susceptibility to infections and certain cancers.
Engineering Effective CAR-T Cells for Immunotherapy
Most successful adoptive cell immunotherapy techniques, especially those utilizing chimeric antigen receptor T cells (CAR-T cells), have focused on enhancing and expanding CD8+ cells. These include breakthrough CAR-T technology like Nanotein Technologies’ NanoSparkTM STEM-T Soluble T Cell Activator which focuses on enriching stemlike CD8+ cells for optimal longevity and effector function for immunotherapies.
However, CD4+ cells have certain beneficial characteristics in engineering better CAR-T cells—such as less susceptibility to activation-induced cell death (AICD) and exhaustion when compared to CD8+ T cells. Nanotein aims to harness the benefits of an increased presence of CD4+ T cells in certain antitumor environments by developing their NanoSparkTM EVEN-T Soluble T Cell Activator to maintain a balanced CD4:CD8 ratio.
Have questions? Contact our team today to learn how Nanotein products can improve your immunotherapy products.