11 The cellular basis of adaptive immune responses

As we saw in a previous chapter, adaptive immune responses are generated by lymphocytes (Fig. 11.1), which are derived from stem cells differentiating within the primary lymphoid organs (bone marrow and thymus). From there, they colonize the secondary lymphoid tissues where they mediate the immune responses to antigens (Fig. 11.2). The lymph nodes are concerned with responses to antigens which are carried into them from the tissues, while the spleen is concerned primarily with antigens which reach it from the bloodstream (Fig. 11.3). Communication between these tissues and the rest of the body is maintained by a pool of recirculating lymphocytes which pass from the blood into the lymph nodes, spleen and other tissues and back to the blood by the major lymphatic channels such as the thoracic duct (Fig. 11.4). This traffic of lymphocytes between the tissues, the bloodstream and the lymph nodes enables antigen-sensitive cells to seek the antigen and to be recruited to sites at which a response is occurring. In addition, unencapsulated aggregates of lymphoid tissue termed ‘mucosa-associated lymphoid tissue’ or MALT, lie in the mucosal surface where they have the job of responding to antigens from the environment, particularly the heavy bacterial load in the intestine, by producing IgA antibodies for mucosal secretions. The lymphocytes which constitute the MALT system recirculate between these mucosal tissues using specialized homing receptors (Fig. 11.5).

Figure 11.1 Lymphocytes and plasma cells. (1) Small B and T lymphocytes have a round nucleus and a high nuclear:cytoplasmic ratio. (2) A large granular lymphocyte with a lower nuclear:cytoplasmic ratio, an indented nucleus and azurophilic cytoplasmic granules. Fewer than 5% of T helper cells, and 30–50% of cytotoxic T cells, γδ T cells and natural killer (NK) cells have this morphology. (3) Antibody formed when B cells differentiate into plasma cells, here stained with fluoresceinated anti-human IgM (green) and rhodaminated anti-human IgG (red) showing extensive intracytoplasmic staining. Note that plasma cells produce only one class of antibody as the distinct staining reveals.

(1 and 2, stained with Giemsa, courtesy of A. Stevens and J. Lowe; 3, adapted from: Zucker-Franklin A. et al. (1988) Atlas of Blood Cells: Function and Pathology, 2nd edn, Vol. 11, Milan: EE Ermes; Philadelphia: Lea and Febiger.)

Figure 11.2 Organized lymphoid tissue. Stem cells (S) arising in the bone marrow differentiate into immunocompetent B and T cells in the primary lymphoid organs. These cells then colonize the secondary lymphoid tissues where immune responses are organized. MALT, mucosa-associated lymphoid tissue.

Figure 11.3 Structure of a lymph node and spleen. (A) Diagrammatic representation of section through a whole lymph node. The cortex is essentially a B-cell region where differentiation within the germinal centres of secondary follicles to antibody-forming plasma cells and memory cells occurs. (B) Diagrammatic representation of spleen showing B- and T-cell areas. (C) Structure of a secondary follicle. A large germinal centre (GC) is surrounded by the mantle zone (Mn). (D) Distribution of B cells in lymph node cortex. Immunochemical staining of B cells for surface immunoglobulin shows that they are concentrated largely in the secondary follicle, germinal centre (GC), mantle zone (Mn), and between the capsule and the follicle – the subcapsular zone (SC). A few B cells are seen in the paracortex (P), which contains mainly T cells. (E) Follicular dendritic cells in a secondary lymphoid follicle. This lymph node follicle is stained with enzyme-labelled monoclonal antibody to demonstrate follicular dendritic cells. (F) Germinal centre macrophages. Immunostaining for cathepsin D shows several macrophages localized in the germinal centre (GC) of a secondary follicle. These macrophages, which phagocytose apoptotic B cells, are called tingible body macrophages (TBM).

(Courtesy of A. Stevens and J. Lowe; C–F reproduced from Male D, Brostoff J, Roth DB, Roitt I. Immunology, 7th edition, 2006. Mosby Elsevier, with permission.)

Figure 11.4 Lymphocyte traffic. The lymphocytes move through the circulation and enter the lymph nodes via the specialized endothelial cells of the postcapillary venules (HEVs). They leave through the efferent lymphatic vessels and pass through other nodes, finally entering the thoracic duct which empties into the circulation at the left subclavian vein (in humans). Lymphocytes enter the white pulp areas of the spleen in the marginal zones; they pass into the sinusoids of the red pulp and leave via the splenic vein.

(Adapted from: Roitt, I. M., Brostoff, J., Male, D. (2002) Immunology, 6th edn. London: Elsevier Science.)

Figure 11.5 Mucosa-associated lymphoid tissue (MALT). Lymphoid cells which are stimulated by antigen in Peyer’s patches (or the bronchi or another mucosal site) migrate via the regional lymph nodes and thoracic duct into the bloodstream and thence to the lamina propria (LP) of the gut or other mucosal surfaces which might be close to or distant from the site of priming. Thus lymphocytes stimulated at one mucosal surface may become distributed selectively throughout the MALT system. This is mediated through specific adhesion molecules on the lymphocytes and the mucosal high-walled endothelium of the postcapillary venules.

(Adapted from: Roitt IM, Brostoff J, Male D. (2002) Immunology, 6th edn. London: Elsevier Science.)

B- and T-cell receptors

B and T cells can be distinguished by their surface markers

As they differentiate into populations with differing functions, B and T cells acquire molecules on their surface that reflect these specializations. It is possible to produce homogeneous antibodies of a single specificity, termed ‘monoclonal antibodies’, that can recognize such surface markers. When laboratories from all over the world compared the monoclonal antibodies they had raised, it was found that groups or clusters of monoclonal antibodies were each recognizing a common molecule on the surface of the lymphocyte. Each surface component so defined was referred to as a ‘CD’ molecule (Table 11.1), where CD refers to a ‘cluster of differentiation’.

Table 11.1 Surface markers on B and T cells

Each lymphocyte expresses an antigen receptor of unique specificity on its surface

Among the surface markers on the B and T cells referred to above are the receptors on the plasma membrane which are used to identify foreign antigens. B cells possess surface immunoglobulin, whereas the T-cell receptor (TCR) on the surface of the T lymphocyte acts as an antigen recognition unit (see Fig. 10.9). We now know that despite the very large number of different components that could be combined together in multiple ways to give a diversity of surface receptors, each B lymphocyte rearranges its germline genes coding for these receptors so that it selects one and only one of the specificities for each receptor polypeptide chain. It then expresses that receptor molecule on its surface (Fig. 11.6). Once this occurs, the other genes coding for these antigen receptors in the lymphocyte are no longer used. In other words, following this genetic rearrangement process, the lymphocyte becomes committed to the synthesis and expression of a single receptor type. An analogous process occurs in the rearrangement of the αβ and γδ genes coding for the TCR. Just as for B cells, each T cell expresses one and only one specific combination of receptor peptides, and therefore shows a single specificity to which it is committed for the whole of its lifespan.

Figure 11.6 Differentiation events leading to the expression of unique IgM monomer sIgM on the surface of an immunocompetent B lymphocyte. There are of the order of 50 germline V_H genes encoding the major portion of the variable region, with 25 minigenes encoding the D segment and six, the J region. As the cell differentiates, V_H, D and J segments on one chromosome randomly fuse to generate lymphocytes with a very wide range of individual heavy chain variable domains. Variable region light chain domains are then formed by random V_L to J recombination. Finally, the variable and constant region genes respectively recombine to encode a single antibody molecule which is expressed on the mature B-cell surface as an sIgM antigen receptor. When activated for antibody production, the transmembrane segment of IgM, which normally holds the molecule on the surface is spliced out at the RNA stage and the soluble form of the IgM is secreted. Subsequently, heavy chain constant region gene switch can occur to generate the various immunoglobulin classes, IgG, IgA, etc. Leader sequences have been omitted for simplicity.

Clonal expansion of lymphocytes

Antigen selects and clonally expands lymphocytes bearing complementary receptors. As there are such a large number of different possible specificities that lymphocytes can express, perhaps of the order of millions, there must of necessity be only a relatively small number of particular specificities to which lymphocytes are committed. Thus, when a microbe invades the body, the total number of lymphocytes initially committed to recognizing the antigens that go to make up a particular microbe is relatively small, and must be expanded to provide a sufficient number to protect the host. Evolution has provided a masterful solution to this problem. When a microbe enters the body, its component antigens combine with only those B lymphocytes whose surface receptors are complementary to the shape of these antigens. The B cells that bind the antigen become activated and proliferate clonally under the influence of soluble growth factors termed cytokines (see section on Cytokines below) to form a large population of cells derived from the original (Fig. 11.7). The majority of these events occur within the lymphoid structure known as a germinal centre (see Fig. 11.3).

Figure 11.7 Generation of a large population of effector and memory cells by clonal proliferation after primary contact of B or T cell with antigen. A fraction of the progeny of the original antigen-reactive lymphocytes become non-dividing memory cells, whereas the others become the effector cells of humoral or cell-mediated immunity. Memory cells require fewer cycles before they develop into effectors, thus shortening the reaction time for the secondary response.

In the case of B cells, a large proportion of the clonally expanded lymphocytes become plasma cells (see Fig. 11.1), dedicated to the synthesis and secretion of antibodies. Since these plasma cells are derived from a parent cell that is already committed to the production of only one specific antibody, the final product is identical to the molecule that was posted on the surface of the original antigen-recognizing cell. Or at least almost so, because somatic mutation of the lymphocytes within the germinal centres which are synthesizing this antibody fine tunes the binding efficiency of the eventual product. The net result is that we have the production of large amounts of antibody which, like that on the surface of the parent cell, can combine with the invading antigen (Fig. 11.7).

A similar process of clonal selection and expansion occurs with T cells, producing a large number of T-cell effectors with the same specificity as the original parent cell; some of these cells release cytokines, whereas others have cytotoxic functions so that they act as effectors of T-cell-mediated immunity. One difference between T and B cells is that the T-cell receptors do not undergo further selection as a result of somatic mutation. Of crucial significance is the fact that in the case of both B and T cells, a fraction of the clonally expanded population differentiates into resting memory cells (Fig. 11.7). Thus, more cells are capable of recognizing the microbial antigen in any subsequent infection than in the initial virgin population that existed before the primary infection occurred. Human memory T cells can be identified by surface markers such as CD45RO, while memory B cells express CD27 and surface IgG, IgA or IgE.