B‐CELLS RESPOND TO THREE DIFFERENT TYPES OF ANTIGEN
There are three main types of B‐cell that respond to infection by secreting antibodies that target specific classes of microbes, with the particular function of each B‐cell subset generally determined by their location. Follicular B‐cells (also called B2 cells) express highly specific monoreactive B‐cell receptors (BCRs), are present in the lymphoid follicles of the spleen and lymph nodes, and typically require T‐cells in order to generate high‐affinity antibodies, and to undergo class switching (Figure 7.21). However, as we shall discuss below, certain types of antigens (called T‐independent antigens) can promote B‐cell activation without the help of T‐cells. The antibodies thus formed are typically of low affinity and do not undergo class switching or somatic hypermutation but provide rapid protection from certain microorganisms and buy time for T‐dependent B‐cell responses to be made. Such rapid antibody responses are mediated by the “innate like” B‐cells; B1 and marginal zone (MZ) B‐cells, which express polyreactive BCRs that are of broad specificity and enable them to recognize multiple different kinds of evolutionarily conserved microbial antigens. In this way, they are similar to the Toll‐like receptors (TLRs) expressed on conventional innate immune cells. Indeed, innate‐like B‐cells also express TLRs and can be directly activated by PAMPs, act as APCs, and secrete cytokines, which places them at the interface between the innate and adaptive immune systems. Importantly, this innate‐like B‐cell response is positioned at strategic areas that are sensitive to microbial invasion, such as the skin, mucosa, and the marginal zone of the spleen, where the lymphatic and circulatory systems converge.
Figure 7.21 Interaction between B‐cells and T‐cells. Scanning electron microscope analysis of a cognate B‐cell/T‐cell pair, embedded in 3‐D collagen matrix.
1. Type 1 thymus‐independent antigens
Certain antigens, such as bacterial lipopolysaccharides, when present at a sufficiently high concentration have the ability to activate a substantial proportion of the B‐cell pool polyclonally (i.e., without reference to the antigen specificity of the surface receptor hypervariable regions). They do this through binding to surface molecules, such as TLRs as discussed in Chapter 1, which bypasses the early part of the biochemical pathway mediated by the specific antigen receptor. At concentrations that are too low to cause polyclonal activation through unaided binding to these mitogenic bypass molecules, the B‐cell population with Ig receptors specific for these antigens will selectively and passively focus them on their surface, where the resulting high local concentration will suffice to drive the activation process (Figure 7.22a).
Figure 7.22 B‐cell recognition of (a) type 1 and (b) type 2 thymusindependent antigens. The complex gives a sustained signal to the B‐cell because of the long half‐life of this type of molecule.
2. Type 2 thymus‐independent antigens
Certain linear antigens that are not readily degraded in the body and that have an appropriately spaced, highly repeating determinant–Pneumococcus polysaccharide, Ficoll, d‐amino acid polymers, and polyvinylpyrrolidone, for example – are also thymus‐independent in their ability to stimulate B‐cells directly without the need for T‐cell involvement. Such antigens persist for long periods on the surface of follicular DCs located at the subcapsular sinus of the lymph nodes and the splenic marginal zone, and can bind to antigen‐specific B‐cells with great avidity through their multivalent attachment to the complementary Ig receptors that they cross‐link (Figure 7.22b).
In general, the thymus‐independent antigens give rise to predominantly low‐affinity IgM responses, some IgG3 in the mouse, and relatively poor, if any, memory. Neonatal B‐cells do not respond well to type 2 antigens and this has important consequences for the efficacy of carbohydrate vaccines in young children.
This innate, T‐cell‐independent detection of microbial antigen is mediated by two types of B‐cell: marginal zone (MZ) B‐cells and B1 B‐cells. MZ B‐cells are located in the marginal zone of the spleen. This specialized area, located at the interface between the circulatory and lymphatic system, acts as a type of filter for blood‐borne pathogens and MZ B‐cells there constantly monitor the circulating levels of PAMP. In contrast, B1 B‐cells are found in the skin and mucosal surfaces, areas continually under siege from pathogens, and act as a rapid first line of defense against microbial invasion. Importantly, activation of both of these innate B‐cell types by simultaneous trigger of BCR and TLRs not only promotes a strong IgM and IgG3 response, but also presents antigen to T‐cells, thus quickly activating the adaptive immune response. Mice specifically deficient in B‐cell Myd88, an essential signal transducer for TLRs, show strong defects in their ability to mount an antibody‐mediated response to many types of infection, suggesting an important role for intrinsic TLR signaling in B‐cell function.
Figure 7.23 T‐helper cells cooperate through protein carrier determinants to help B‐cells respond to hapten or equivalent determinants on antigens (Ag) by providing accessory signals. (For simplicity we are ignoring the MHC component and epitope processing in T‐cell recognition, but we won’t forget it.)
3. Thymus‐dependent antigens
The need for collaboration with T‐helper cells
Many antigens are thymus‐dependent in that they provoke little or no antibody response in animals that have been thymectomized at birth and therefore have few T‐cells (Milestone 7.1). Such antigens cannot fulfill the molecular requirements for direct stimulation: they may be univalent with respect to the specificity of each determinant; they may be readily degraded by phagocytic cells; and they may lack mitogenicity. If they bind to B‐cell receptors, they will sit on the surface just like a hapten and do nothing to trigger the B‐cell (Figure 7.23). Cast your mind back to the definition of a hapten – a small molecule such as dinitrophenyl (DNP) that binds to preformed antibody (e.g., the surface receptor of a specific B‐cell) but fails to stimulate antibody production (i.e., stimulate the B‐cell). Remember also that haptens become immunogenic when coupled to an appropriate carrier protein. Building on the knowledge that both T‐ and B‐cells are necessary for antibody responses to thymus‐dependent antigens (Milestone 7.1), we now know that the carrier functions to stimulate T‐helper cells that cooperate with B‐cells to enable them to respond to the hapten by providing accessory signals (Figure 7.23). It should also be evident from Figure 7.23 that, while one determinant on a typical protein antigen is behaving as a hapten in binding to the B‐cell, the other determinants subserve a carrier function in recruiting T‐helper cells.
Figure 7.24 T‐ and B‐cell interaction in a B‐cell follicle. Multiple Tcell (red) and B‐cell (green) pairs form at the T zone border within a B‐cell follicle (arrowheads).
Figure 7.25 B‐cell handling of a thymus‐dependent antigen and presentation to an activated T‐cell. Antigen captured by the
surface Ig receptor is internalized within an endosome, processed, and expressed on the surface of the B‐cell with MHC class II (see Figure 5.16). Co‐stimulatory signals through the CD40–CD40L (CD154) interaction are required for the activation of the resting B‐cell by the T‐helper cell. In addition to CD40L‐based co‐stimulation, helper T‐cells also provide additional stimulation to the B‐cell in the form of cytokines such as IL‐4.
Antigen processing by B‐cells
The need for physical linkage of hapten and carrier strongly suggests that T‐helpers must recognize the carrier determinants on the responding B‐cell in order to provide the relevant accessory stimulatory signals. However, as T‐cells only recognize processed membrane‐bound antigen in association with MHC molecules, the T‐helpers cannot recognize native antigen bound simply to the Ig receptors of the B‐cell as naively depicted in Figure 7.23. All is not lost, however, as primed B‐cells can present antigen to T‐helper cells (Figure 7.24) – in fact, they work at much lower antigen concentrations than conventional presenting cells because they can focus antigen through their surface receptors. Antigen bound to surface Ig is internalized in endosomes that then fuse with vesicles containing MHC class II molecules with their invariant chain. Processing of the protein antigen then occurs as described in Chapter 5 (see Figure 5.16) and the resulting antigenic peptide is recycled to the surface in association with the class II molecules, where it is available for recognition by specific T‐helpers (Figure 7.25 and Figure 7.26). The need for the physical union of hapten and carrier is now revealed; the hapten leads the carrier to be processed into the cell, which is programmed to make anti‐hapten antibody and, following stimulus by the T‐helper‐recognizing processed carrier, it will carry out its program and ultimately produce antibodies that react with the hapten (is there no end to the wiliness of nature?).
Figure M7.1.1 The antibody response to some antigens is thymus dependent and, to others, thymus independent. The response to tetanus toxoid in neonatally thymectomized animals could be restored by the injection of thymocytes
Figure M7.1.2 The antibody response to a thymus‐dependent antigen requires two different lymphocyte populations. Different populations of cells from a normal mouse histocompatible with the recipient (i.e., of the same H‐2 haplotype) were injected into recipients that had been X‐irradiated to destroy their own lymphocyte responses. They were then primed with a thymus‐dependent antigen such as sheep red blood cells (i.e., an antigen that fails to give a response in neonatally thymectomized mice; Figure M7.1.1) and examined for the production of antibody after 2 weeks. The small amount of antibody (Ab) synthesized by animals receiving bone marrow alone is due to the presence of thymocyte precursors in the cell inoculum that differentiate in the intact thymus gland of the recipient.
Milestone 7.1 T–B collaboration for antibody production
In the 1960s, as the mysteries of the thymus were slowly unraveled, our erstwhile colleagues pushing back the frontiers of knowledge discovered that neonatal thymectomy in the mouse abrogated not only the cellular rejection of skin grafts, but also the antibody response to some but not all antigens (Figure M7.1.1). Subsequent investigations showed that both thymocytes and bone marrow cells were needed for optimal antibody responses to such thymus‐dependent antigens (Figure M7.1.2). By carrying out these transfers with cells from animals bearing a recognizable chromosome marker (T6), it became evident that the antibody‐forming cells were derived from the bone marrow inoculum, hence the nomenclature “T” for thymus‐derived lymphocytes and “B” for antibody‐ forming cell precursors originating in the bone marrow. This convenient nomenclature has stuck even though bone marrow contains embryonic T‐cell precursors, as the immunocompetent T‐ and B‐cells differentiate in the thymus and bone marrow, respectively.