Role of CD4-positive T cells in Bacterial Killing

Aims and Objectives

1. Know the major subpopulations of T lymphocytes.
2. Understand how T lymphocytes recognize antigen.
3. Understand the role that CD4+ T lymphocytes play in the killing of intracellular micro-organisms.

T lymphocytes play a major role in defense against intracellular pathogens such as viruses, protozoa and intracellular bacteria, and are also involved in immunity to extracellular pathogens by providing "help" for the antibody response. This page provides a general introduction to the biology of T lymphocytes, and will describe the role that the CD4+ subpopulation of T lymphocytes plays in the elimination of intracellular micro-organisms such as Mycobacteria (e.g. in tuberculosis and leprosy) and Candida.

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Introduction to T Lymphocytes

The process of negative selection is designed to delete T cells whose antigen receptors recognise self antigens with high affinity, and would therefore be autoreactive if released into the periphery (ie negative selection is the main mechanism by which "self tolerance" is achieved), whilst positive selection ensures that cells which recognise "foreign" antigens survive. Only T cells which survive these phases are released from the thymus as mature, antigen reactive T cells.

In the periphery, mature T cells can be divided into several major subpopulations, based on the expression of different cell surface marker molecules

All T cells express a surface molecule called CD3. Expression of the CD3 surface marker is specific for T lymphocytes, and is often used to characterise T cells. Like B cells, T lymphocytes express an antigen specific surface receptor (TcR):

Expression of the TcR confers antigen specificity to the T cell. The nature of the TcR is distinct from the antigen receptor (surface immunoglobulin) on B lymphocytes, but the diversity of TcR expressed is achieved by a process of TcR gene rearrangements similar to the rearrangment of V, D and J segments of immunoglobulin genes seen in B cells. This topic will not be covered in the lecture, but students will be expected to read it for themselves. There are two forms of TcR - the great majority of T cells in the periphery (>90%) express alpha/beta TcR, whilst a minority of T cells express gamma/delta TcR (the Greek letters refer to the polypeptide chains from which the TcR are formed). The function of gamma/delta T cells in the immune response is not well understood, and these cells will not be discussed further. Peripheral alpha/beta TcR positive cells express either the CD4 or the CD8 surface marker, and these markers can be used to define the major subpoplations of mature T cells in the periphery:

In broad terms, the CD4+ T lymphocytes represent the "helper" T cell population. These cells are discussed in more detail in the second part of this lecture. The CD8+ T lymphocytes represent the antigen specific cytotoxic T lymphocytes (CTL), which respond to and kill cells which are infected with intracellular pathogens such as viruses, some intracellular bacteria (e.g. Listeria ) and some intracellular protozoa (eg malaria parasites). The function of CD8+ T cells is covered in detail in the lecture on antiviral immunity.

In contrast to immunoglobulin on B cells or soluble antibody molecules, T cells do not recognise free soluble or surface bound antigen, but require the antigen to be processed and presented by an "antigen presenting cell". T lymphocytes (via their TcR) recognise antigen in the form of short peptides presented in association with "self" class I or class II major histocompatibility complex (MHC) molecules at the surface of an antigen presenting cell (APC)

This phemomenon is known as "MHC restriction" of T lymphocyte responses. Students should familiarise themselves with the MHC (in man termed HLA, for Human Leukocyte Antigen), and the role of MHC molecules in antigen presentation to T lymphocytes.

In humans, the MHC genes are termed HLA -A,-B and -C for MHC class I, and HLA - DR, -DP and -DQ for MHC class II. These genes are highly polymorphic, and each individual inherits 2 copies of each gene. The degree of polymorphism is such that most individuals will express genes encoding 6 different HLA class I molecules (i.e. 2x -A; 2x -B and 2x -C), and 6 different HLA class II alleles (2x -DR, 2x -DP and 2x -DQ). Different HLA molecules bind a different set of peptides, so the polymorphism within MHC genes maximises the number of different (antigenic) peptides which can be bound and presented to an individual's T cells.

T cell recognition of antigen involves direct cell-cell contact between the antigen-specific TcR on the T lymphocyte and an MHC/peptide complex at the surface of a MHC compatible antigen presenting cell. In general, class I MHC molecules present antigen to CD8+ T cells, and class II MHC molecules present antigen to CD4+ T cells. Class I MHC molecules are expressed constitutively on almost all nucleated cells of the body, whilst constitutive expression of class II molecules is restricted to certain cells of the immune system - B cells, macrophages and dendritic cells - although expression of class II MHC may be induced on other cell types at sites of inflammation. There are distinct pathways of processing of antigen for presentation in association with class I and class II MHC molecules. Antigens bound for presentation in association with MHC class I molecules are derived from the cell's cytosol, and are usually endogenously synthesised within the cell (for instance, viral antigens):

In contrast, the majority of antigens which are presented by MHC class II molecules are derived from exogenous antigens, such as soluble proteins or extracellular organisms:

The Role of CD4+ T Lymphocytes in Infection with Intracellular Organisms

CD4+ T cells recognise antigen which has been processed and is presented in association with a self class II MHC molecule. In initiating the immune response, this interaction takes place in the lymphoid tissue, and involves T cell interaction with so called "professional antigen presenting cells" -B cells, macrophages and dendritic cells - which take up, process and present the relevant antigen. Once a CD4+ T cell has been activated in this way, it is capable of recognising the antigen presented by any cell which expresses the appropriate class II MHC molecule. In this way, initial activation of the immune response is controlled within the lymphoid tissues, whilst primed CD4+ T cells are then able to respond to antigen presented at distant sites within the body.

CD4+ T cells act by releasing cytokines in response to antigenic stimulation. Cytokines are soluble intercellular messenger molecules, which interact with specific receptor molecules on their "target" cells. The release of cytokines allows cells of different types to "talk" to each other in the on-going immune response. A wide range of cytokines are involved in the immune response, each of which has a specific set of activities on its target cell(s). The table below lists some of the more important cytokines involved in the cell mediated immune response, together with their main effects and target cell types. By releasing cytokines in response to antigenic stimulation, CD4+ T cells are able to orchestrate an appropriate cell mediated immune response to infection.

Principle Cytokines Involved In Cell Mediated Immunity

One of the major effector functions of CD4+ T cells is in the activation of macrophages. Macrophage activation plays an important role in enhancing bacterial killing at sites of infection. Resting macrophages are relatively poor at phagocytosing microrganisms, show little respiratory activity (involved in the killing of phagocytosed or intracellular organisms), and are only weakly able to initiate T cell activation. However, in response to cytokines (principally gamma interferon) released by antigen-responsive effector T cells, macrophages become "activated", and have greatly enhanced phagocytic capability, increased respiratory activity (and therefore increased killing of intracellular organisms), increased levels of MHC class I and class II expression, and enhanced antigen presenting capacity. In addition, activated macrophages release cytokines and growth factors which promote inflammation. In general, macrophage activation involves the upregulation or de novo expression of a variety of genes. Killing of micro-organisms by macrophages involves phagocytosis of the organism, followed by the production of oxygen radicals which then kill the phagocytosed organisms. Macrophage activation results in upregulation of the high affinity Fc receptor for immunoglobulin, promoting phagocytosis of opsonised bacteria, and increased expression of a cytochrome enzyme that catalyses the reaction that generates free oxygen radicals involved in bacterial killing.

The interaction between antigen specific CD4+ T cells and macrophages forms the basis of the delayed type hypersensitivity response, which is one of the main cell mediated immune effector mechanisms in eliminating infections. This form of cell mediated immunity plays an important role in immunity to infections with intracellular organisms such as Mycobacteria and Candida.

The delayed type hypersensitivity (DTH) reaction may also be used in the investigation of certain patients, when used as a skin test for evidence of prior sensitization to an antigen. For example, a DTH skin reaction to purified protein derivative of Mycobacterium tuberculosis forms the basis of the Heaf or Mantoux tests for tuberculosis. The presence of a positive skin test correlates with prior exposure to M.tb. or BCG vaccination, and to a degree with protective immunity against M.tb. A very strongly positive reaction may indicate infection with M. tb. The DTH skin test is negative in patients who have not been exposed to M.tb. or immunised with BCG. It may become negative in patients with miliary tuberculosis in whom infection is spreading out of control, indicating the loss of protective immunity and an inability to contain the infection. Most adults will have encountered, and will give positive DTH skin tests to a range of common T cell recall antigens (such as PPD, Candida, Trichophyton). In certain disease conditions, most notably in sarcoidosis, this skin test reactivity to a range of common recall antigens is lost. The loss of responsiveness is known as immunological "anergy", and may be specific for a particular antigen / organism (eg for PPD in miliary tuberculosis) or more generalised (as in sarcoidosis).

Helper T cells can be further divided into several broad groups, based on the specific profiles of cytokines which they release. These groups are known as Th1, Th2 and Th0 cells. Th1 cells release predominantly IL-2 and gamma interferon in response to antigenic stimulation. These cytokines promote T cell proliferation, macrophage activation and the DTH response, and enhance the cytolytic activity of NK cells and antigen specific CTL (this is sometimes termed a pro-inflammatory type of response). In contrast, Th2 cells release predominantly IL-4, IL-5 and IL-10 in response to antigen. IL-4 and IL-5 mediate antibody isotype switching towards

IgE or IgA responses, and promote eosinophil recruitment, skewing the immune response towards an "allergic" type of response. Th0 cells release a set of cytokines with characteristics of both Th1 and Th2 types of response. In fact, rather than viewing helper T cells as falling into two or three distinct groups, it is probably more accurate to view them as forming a continuous spectrum, with Th1 and Th2 cells at opposite ends of the scale, and Th0 cells taking up the middle ground.

The distinction between Th1 and Th2 cells may, however, be clinically relevant in combating infection. This is illustrated by the disease leprosy, which is caused by infection with Mycobacterium leprae. Clinically, leprosy presents a spectrum of disease, with two extreme forms, tuberculoid and lepromatous leprosy. In tuberculoid leprosy there is less tissue damage, fewer viable organisms and a strong delayed type hypersensitivity response to the infection, with T cell dependent granuloma formation consisting of activated macrophages, T cells and epithelioid cells, and containment of the organism. Immunologically the response is characteristic of a Th1 type of response. In lepromatous leprosy, the DTH response is suppressed, antibody levels are raised but do not control the infection, and there are large numbers of organisms invading the tissues. The T cell response in lepromatous leprosy has the characteristics of a Th2 type response. In contrast, however, in parasitic infections (for example infections with nematodes and flukes) a Th2 type of response may be beneficial, as the main effectors of the immune system which combat the infecting organism are IgE and eosinophils.

From these examples, the importance of CD4+ T cells and macrophages in protective immunity and killing of intracellular organisms such as M.tb. and Candida should be evident, but in some cases the "wrong type" of T cell response may be of little benefit to the host, or even harmful. A lot of current research in immunology focusses on how preferentially to stimulate the most appropriate type of CD4+ helper T cell response, or how to shift the balance of an ongoing response in one direction or another.

© AJC 2007.