Fundamentals of Immunology - Lecture Course by M. V. Skok - Kyiv 2002

Chapter II. Mechanisms of the Immune Response

Lecture 9. Cytokines and the Regulation of the Immune Response

This lecture will focus on specific Examples of cytokine involvement in T lymphocyte activation and the Regulation of the Immune Response. As noted in the introductory lecture, T lymphocytes are divided into two main subtypes: T helper Cells (Th) and T killer cells, or cytotoxic T lymphocytes (CTLs). Th cells express the CD4 coreceptor on their membrane, which serves as their specific marker, recognize antigen in the context of MHC Class II molecules, and play a regulatory role in the immune response by activating B lymphocytes to differentiate into plasma cells and secrete Antibodies, as well as priming CTL precursors for cytotoxic action. CTLs express the CD8 coreceptor, recognize antigen in the context of MHC class I molecules, and, upon receiving a signal from Th cells, execute a cytotoxic effector function by destroying infected cells. Consequently, the activation of Th and CTL cells requires distinct activation signals and triggers different metabolic pathways.

1. Activation of Th cells.

Prior to encountering an antigen, Th cells exist in a resting state. These so-called small lymphocytes can survive in the body for years. They express an antigen-specific receptor (T-Cell receptor) on their surface, the CD45 phosphatase located near the T-cell receptor without being associated with it, low levels of CD40L, and a low-affinity IL-2 receptor composed of β and γ subunits.

Upon encountering an antigen presented with MHC class II Proteins, the T lymphocyte initiates expression of the Gene encoding the IL-2 receptor α-subunit, resulting in The formation of a high-affinity receptor. Its interaction with exogenous IL-2 drives the T lymphocyte through the Cell Cycle, prompting it to divide and form a clone. Furthermore, interaction with IL-2 leads to Upregulation of CD40L expression. Subsequent interaction of this cell with a B lymphocyte enhances the expression of costimulatory molecules B7.1 and B7.2 on the B cell. The binding of B7.1 to CD28 On the surface of the T lymphocyte results in the stabilization of IL-2 mRNA, enhanced IL-2 synthesis, and increased METABOLISM/31.html">Transcription of the IL-2 gene. As a result, IL-2 secretion increases approximately 100-fold, enabling The Cell to activate both itself (in an autocrine manner) and neighboring cells (in a paracrine manner). The interaction between the T-cell receptor and the antigen also induces Conformational Changes in the LFA-1 molecule, which strengthens the adhesive bond between the T lymphocyte and the antigen-presenting cell.

As a result of all these interactions, the T cell transitions into an activated state within 4-5 days. This state is characterized by elevated expression levels of adhesion and costimulatory molecules, as well as the expression of a distinct CD45 isoform—CD45RO—generated via alternative mRNA splicing. This form of CD45 associates with the T-cell receptor and facilitates the transmission of the activation signal from the antigen. Consequently, the T cell no longer requires costimulatory signals to respond to the antigen.

Thus, driven by the antigen, naive T lymphocytes initially proliferate and subsequently transform into activated cells. Their subsequent functioning depends on THE SPECTRUM OF cytokines produced under the Influence of the antigen.

2. Populations of Regulatory T Lymphocytes.

As discussed in previous lectures, the type of immune response—cellular or humoral—is determined by The Nature of the antigen and the characteristics of its Processing. However, most natural pathogens, including Bacteria, Viruses, and parasites, are complex Antigens containing both endogenous and exogenous elements. A crucial regulatory mechanism in determining whether the immune response will proceed via a humoral or cellular pathway is the participation of specific helper T cell subsets, namely T helper 1 (Th1) and T helper 2 (Th2) cells.

This Conclusion was reached through The Study of murine leishmaniasis. It was discovered that two inbred mouse strains exhibited different susceptibilities to Leishmania infection: BALB/c mice developed the disease, whereas C57BL/6 mice did not. Analysis of their Blood revealed distinct levels of two key cytokines: BALB/c mice had high levels of IL-4 and low levels of interferon-gamma (IFN-γ), whereas C57BL/6 mice showed the reverse pattern—low IL-4 and high IFN-γ levels.

A similar pattern was observed in human leprosy. The disease can manifest in a lepromatous form, characterized by active tissue destruction and high loads of intracellular parasites, with patients exhibiting elevated IL-4 levels. Conversely, in the tuberculoid form of leprosy, few parasites are detected, and tissue damage is driven by immune-mediated inflammation; such patients show high levels of IFN-γ.

Long-term cloning of CD4⁺ T cells in culture revealed two distinct clone types differing in their cytokine secretion profiles: one subset secreted IFN-γ, IL-2, and TNF-α (tumor necrosis factor), while the other secreted IL-4, IL-5, IL-6, IL-10, and IL-13. These were designated as Th1 and Th2 cells, respectively (Fig. 18).

It turned out that Th1 and Th2 cells perform distinct Functions. Th1 cells stimulate macrophage microbicidal activity and cellular Immunity (CTLs); they do not stimulate resting B cell activation, but in activated B cells, they promote the secretion of IgG2a, thus functioning in the secondary HUMORAL IMMUNE RESPONSE. Th2 cells stimulate B cell proliferation and differentiation, as well as class switching from IgM to IgE, IgG1, and IgG3. They operate in both Primary and secondary humoral immune responses.

Thus, the presence of Th1 cells in C57BL/6 mice and patients with the tuberculoid form of leprosy facilitated the eradication of intracellular parasites via cellular immunity, whereas in BALB/c mice and patients with the lepromatous form, the presence of Th2 cells promoted a humoral immune response without clearing the infected cells.

The differentiation of a precursor into a Th1 or Th2 cell depends on the cytokine milieu present during priming. Specifically, a naive Th precursor arising from the Activation of a resting T lymphocyte (CD4⁺, secreting IL-2) differentiates into a Th0 cell, which secretes both IL-4 and IFN-γ. Upon encountering an antigen that triggers the secretion of specific cytokines, the pathway diverges into Th1 or Th2 lineages. Infected macrophages secrete IL-12, which, together with IL-2, promotes Th1 development (a process inhibited by IL-4). Antibody-opsonized antigens bind to Fc receptors on basophils, mast cells, and CD4⁺ T cells, which serve as a source of IL-4 that drives Th2 differentiation (while IFN-γ inhibits this process).

Thus, the developmental pathway of the immune response—whether cellular or humoral—depends on which lymphokine secretion is triggered by a given antigen. At THE MOLECULAR LEVEL, this boils down to the selective transcription of the IL-4 or IFN-γ genes. Bacterial antigens predominantly activate Th1 responses, whereas helminth infections preferentially drive Th2 responses. Typically, infections provoke responses that are not exclusively Th1 or Th2. For instance, the Influenza virus stimulates The production of both IFN and IL-10 while suppressing IL-5 and IL-4 production. Furthermore, the choice between Th1 and Th2 pathways is genetically determined, as demonstrated by different mouse strains infected with Leishmania.

For Th1 development, IL-4 produced by Th2 cells acts as a negative signal. Conversely, Th2 development requires IL-4, whereas IFN-γ produced by Th1 cells acts as an inhibitory signal. This means that Th1 and Th2 cells are mutually antagonistic: Th1 activity suppresses Th2 development, and Th2 cells inhibit Th1 differentiation. In their mature state, Th1 and Th2 cells continue to exert mutual suppression.

The Molecular Basis of this antagonism is not yet fully understood. It is known, for example, why Th2 cells are inhibited by IFN-γ while Th1 cells are not. IFN-γ interacts with a specific receptor composed of α and β chains. This receptor binding activates the Tyrosine Kinases Jak1 and Jak2, which phosphorylate Stat1. Stat1 then homodimerizes, migrates to The Nucleus, and drives the transcription of the IFN-γ gene. In Th1 cells, the β-chain of the IFN-γ receptor is absent. Consequently, Th1 cells utilize receptor modification to prevent the activation of the IFN-γ gene, thereby rendering themselves resistant to this cytokine.

The mutual antagonism between Th1 and Th2 cells has helped explain many phenomena that were attributed to T suppressor cells in the 1970s. According to modern understanding, depending on the context, the exact same T cells can act as either activators or suppressors, with cytokines such as IL-4 or IFN-γ acting as the effectors of their action.

A prime example of such interactions is the response to Leishmania infection. Th1 cells secrete IFN-γ and TNF-α, which activate macrophages. Macrophages synthesize nitric oxide (NO), a potent toxic agent against intracellular pathogens. Th2 cells synthesize IL-4 and IL-10, which suppress NO synthase expression and promote the progression of leishmaniasis (for example, in BALB/c mice). Thus, in this scenario, Th2 cells function as suppressors.

Therefore, the primary determinant of the immune response is the antigen. Depending on how it gains entry into immune system cells or target cells (endogenously or exogenously), it may be presented via MHC class I or class II molecules, which represents the first critical checkpoint in The Development of a humoral or cellular response. The nature of processing also depends on antigen dose: as we recall, an excess of endogenous antigen can also be presented via MHC class II, while an excess of exogenous antigen can be presented via MHC class I.

Depending on the type of antigen, its route of entry, and its dose, specific regulatory T cells are activated, which—through their secreted cytokines—set the stage for either a humoral or cellular response, marking the second critical checkpoint. Th1 cells activate CTL precursors, whereas Th2 cells activate B lymphocytes. Their cytokines also stimulate the upregulation of costimulatory molecules on T AND B lymphocytes.

Summary.

1. The primary factor regulating the immune response is the antigen, which determines whether the immune reaction will proceed via a cellular or humoral pathway.

2. The key regulatory cell is the CD4⁺ T lymphocyte. Depending on the type of cytokines produced, CD4⁺ T lymphocytes differentiate into Th1 and Th2 cells. Th1 cells stimulate the cell-mediated cytotoxic response, whereas Th2 cells promote the humoral response. They develop in cytokine microenvironments with distinct profiles and mutually inhibit each other.

3. At THE CELLULAR LEVEL, regulation involves the control of interleukin expression, their corresponding receptors, and costimulatory molecules.

4. At the molecular level, regulation alters the nature of signal Transduction from the cell surface to the nucleus.