IMMUNOLOGY - Roitt I. - Mir 2000

Chapter 11. Cell Interactions in the Humoral Immune Response

THE HUMORAL IMMUNE RESPONSE IN VIVO

Early studies of the humoral Immune Response focused primarily on measuring specific Antibodies in the Blood of animals following immunization with T-dependent or T-independent Antigens. As data on B-lymphocyte development and maturation accumulated, the cellular mechanisms underlying immune reactions in vivo began to emerge. The key Features of the humoral response associated with these cellular Functions include:

✵ enhanced antibody production during the secondary response,

✵ Ig isotype switching,

✵ affinity maturation of antibodies, and

✵ formation of immunological memory. Some of these processes only become clear when the heterogeneous B-Cell population is viewed as an integrated system rather than merely a collection of individual B Cells. The elements of the humoral response in vivo are detailed below.

Following primary antigen stimulation, the immune response enters its initial phase—the lag phase—during which no antibodies are detectable in the blood. This is followed by the logarithmic growth phase of the antibody titer, a plateau phase, and a decline phase. The decrease in antibody levels results either from their Catabolism or from their binding to the antigen and subsequent clearance from the Circulation (Fig. 11.21).

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Fig. 11.21. The humoral immune response induced by antigenic stimulation proceeds in 4 phases.

1. The lag phase, during which no antibodies are detectable in the serum.

2. The log phase, during which the antibody titer increases logarithmically.

3. The plateau phase, characterized by stabilization of the antibody titer.

4. The decline phase, during which IMMUNOGLOBULINS are catabolized or cleared.

The time intervals between these phases and the level of antibodies produced depend on The Nature of the antigenic stimulus and the physiological state of the Organism.

Comparing the responses to Primary and secondary antigenic stimulation reveals 4 fundamental differences between them.

Time factor. The secondary immune response is characterized by a shortened lag phase and significantly extended plateau and decline phases.

Antibody titer. The antibody level during the plateau phase is substantially higher in the secondary response, typically exceeding the antibody content following primary immunization by a factor of 10 or more.

Antibody class. The primary immune response predominantly produces IgM antibodies, whereas the secondary response is dominated by IgG, with IgM present only in very small amounts.

Antibody affinity. As a rule, antibodies produced during a secondary immune response exhibit significantly higher affinity. This characteristic is known as affinity maturation.

The CHARACTERISTICS OF THE primary and secondary immune responses are illustrated in Fig. 11.22.

Fig. 11.22. Compared to the primary humoral response, the same antibody concentration is reached more rapidly and maintained longer following secondary stimulation, yielding a higher titer dominated by IgG. (In the primary response, the appearance of IgG is preceded by IgM synthesis.)

The presence of antibody-forming cells (AFC) in the Spleen of immunized animals can be detected using the plaque-assay technique (see Ch. 29). Research shows that AFCs appear in this organ approximately 24 hours before antibodies become detectable in measurable titers in the blood serum.

Ig Isotype Class Switching is T-Cell Dependent

During a T-dependent immune response, There is a gradual shift in the class of prevailing specific antibodies, typically toward the dominance of IgG. When immunized with T-independent antigens, this immunoglobulin isotype switching does not occur, and IgM generally remains the main class of antibodies produced.

The isotype switch from IgM to IgG is not a random event. The subclasses of IgG produced by plasma cells vary depending on the Nature of the stimulus. For instance, in mice, the administration of complete Freund's adjuvant leads predominantly to The production of IgG2 antibodies, whereas using protein antigens precipitated with alum for immunization elicits primarily an IgG1 response. Switching to the synthesis of IgA or IgE may also occur; cells producing immunoglobulins of these isotypes are concentrated mainly in the lymphoid Tissues of mucous membranes.

The Molecular Basis of isotype switching has been studied in considerable detail. DNA rearrangements depend on signals generated by CD4⁺ T cells, cytokines, and CD40L, which plays a particularly crucial role. T-cell cytokines present in the immediate vicinity of B cells determine the new isotype of the immunoglobulins produced. The cytokine IL-4 promotes switching to the synthesis of IgG1 and IgE, whereas TGFβ (synthesized not only by T cells) induces switching to the production of IgA and IgG2b, and IFNγ stimulates The formation of IgG2a. Cytokines can also suppress the expression of certain isotypes; for example, IL-4 inhibits the synthesis of IgG2a.

During isotype switching, cytokine-dependent METABOLISM/31.html">Transcription of DNA occurs in a new constant region, modified through a mechanism known as switch recombination. This process involves specific "switch regions" located at the 5' end of each Gene encoding the constant region domains of heavy chains of different isotypes recombining with each other, while the intervening DNA segment is deleted (Fig. 11.23).

Fig. 11.23. Organization of human immunoglobulin heavy chain (IGH) genes. Initially, VDJ and heavy µ-chain Genes are transcribed in B cells; splicing results in the formation of mRNA for IgM. Under The Influence of T cells and cytokines, an Ig class switch may occur, in this case from the synthesis of IgM to the production of IgG2. A "switch region" is located upstream of each heavy chain gene, except for CD (which encodes IgD). During switching, these regions recombine with each other, while the intervening C genes—in this case CM, CD, CG3, CG1, and CA1—are lost. (Pseudogenes are not shown in this diagram.)

Affinity Maturation Depends on Cell Selection

In a secondary Immune Response to T-dependent antigens, antibodies possess, on average, a higher affinity than in a primary response. This is associated with the switch in antibody synthesis from IgM to IgG (no increase in affinity is observed during an IgM response).

The degree of affinity increase is inversely proportional to the dose of administered antigen: when high doses are introduced, affinity increases less than after immunization with low doses of antigen (Fig. 11.24). It has been suggested that at low antigen concentrations, only those B cells possessing high-affinity receptors bind it in sufficient quantities; this binding induces the proliferation and differentiation of these B cells. At significant antigen concentrations, however, both high- and low-affinity B cells bind the antigen and become activated.

Fig. 11.24. Average affinity of IgM and IgG antibodies produced following primary and secondary administration of a T-dependent antigen. IgM affinity remains constant throughout the entire response. Affinity maturation in the IgG response depends on the dose of the re-administered antigen. Using low doses of antigen produces higher-affinity antibodies than administering high doses, because high-affinity clones successfully outcompete others for a limited amount of antigen.

Although individual B cells generally do not change their primary Specificity, the affinity of antibodies produced by a clone can undergo changes As a result of somatic hypermutation of recombined antibody genes (see Chapter 8). It appears that two processes contribute to affinity maturation:

✵ the generation of higher-affinity B-cell clones through minor Changes in the Structure of antibodies produced by daughter cells; such changes appear at a late stage of the primary immune response to a T-dependent antigen;

✵ the selective expansion of high-affinity clones under the influence of antigen.

Somatic hypermutation occurs in antibody-producing cells during a T-dependent immune response and plays a vital role in the production of high-affinity antibodies. In this regard, it represents a normal and beneficial process. However, this same process occasionally leads to the generation of high-affinity IgG autoantibodies, such as anti-DNA antibodies, capable of causing severe disorders. Such mutation has been experimentally detected in long-term tissue culture, but its role in the onset of common autoimmune diseases remains unknown.

B-Cell Activation Takes Place in Germinal Centers

The process of B-cell affinity maturation unfolds within germinal centers (Fig. 11.25). These structures form in the spleen or Lymph Nodes several days after antigenic stimulation. B lymphocytes activated by T cells through the binding of CD40 to its Ligand migrate into primary follicles, where a dense network of follicular dendritic cells resides. Within this microenvironment, rapid B-Cell Division occurs, accompanied by somatic mutation of Ig genes. B cells with high-affinity receptors undergo survival selection based on the interaction of their membrane-bound surface antibodies and the B-cell-coreceptor complex with antigen and Complement On the surface of follicular dendritic cells. Upon passing through the germinal center, B lymphocytes express the "cell survival" gene, bcl-2. Cells with high-affinity IgG evade apoptosis through the action of the bcl-2 product, whereas cells with low-affinity receptors lack this property and die by apoptosis.

Fig. 11.25. B cells enter the germinal center and proliferate rapidly. Proliferation is accompanied by hypermutation of immunoglobulin genes. Antigen presented by follicular dendritic cells (FDCs) is bound only by those B lymphocytes that possess high-affinity antigen-specific receptors. These B cells express the bcl-2 gene and do not undergo apoptosis thanks to interaction with T cells, wherein B cells act as antigen-presenting cells for T lymphocytes. This interaction initiates T-dependent immunoglobulin class switching, which is determined in part by T cells present in secondary lymphoid tissue as well as the type of immune response (Th1 or Th2). B lymphocytes leave the germinal centers and differentiate into either plasma cells or memory B cells (Bm).

Immunological Memory

The organism's capacity to mount a secondary immune response is based on the property of "immunological memory." Vaccination owes its efficacy to this exact same property.

Cells serving the function of immunological memory accumulate as a result of the expansion of antigen-specific lymphocyte populations during the primary immune response—that is, an increase in the number of resting B AND T cells capable of responding to subsequent exposure to the same antigen. Memory B cells differ qualitatively from naive B lymphocytes in that they begin producing IgG antibodies sooner and typically possess higher-affinity antigen receptors due to selection during the primary immune response.

Memory T cells are unlikely to possess higher-affinity receptors compared to naive T cells, since T lymphocytes do not undergo hypermutation. However, memory T cells are capable of responding to lower doses of antigen, which suggests that their receptor complex as a whole (including adhesion molecules) functions more efficiently. It is now well established that immunological memory is determined not merely by the accumulation of populations of identical cells; the Properties of Individual cells also change, as evidenced by alterations in the expression of cell surface molecules and cytokines. CD4⁺ memory T cells produce cytokines more rapidly and vigorously.

Questions for Consideration

■ What are the differences in T-cell interactions with macrophages, dendritic cells, and B lymphocytes?

■ How do affinity maturation and Ig class switching determine the type and efficacy of an immune response? What is The Role of T cells in regulating these processes?

■ What induces lymphocyte proliferation, and how is it regulated?

■ Can cytokines cause harmful effects?

■ The gld mutation affects the CD90L molecule, which is essential for lymphocyte apoptosis in peripheral Lymphoid Organs. What effect might this mutation have on B-cell development in lymph nodes?

Introduction/47.html">Further Reading

Brodsky F.M., Guagliardi L. 1991. The Cell biology of ANTIGEN Processing AND presentation. Annu. Rev. Immunol. 9: 707-44.

Cantrell D. 1996. T CELL ANTIGEN receptor signal Transduction pathways. Annu. Rev. Immunol. 14: 259-74.

Clark E.A., Ledbetter J.A. 1994. How B and T cells talk to each other. Nature 367: 425-26.

Fearon D.T., Carter R.H. 1995. The CD19, CR2AAPA-1 complex of B lymphocytes: linking natural to acquired Immunity. Annu. Rev. Immunol. 13: 127-49.

Feldmann M., Brennan F.M., Maini R.N. 1996. Role of cytokines in rheumatoid Arthritis. Annu. Rev. Immunol. 14: 397-40.

Foy T.M., Aruffo A., Bajorath J. et al 1996. Immune regulation by CD40 and its ligand CD39. Annu. Rev. Immunol. 14: 591-617.

Lane P. 1996. Development of B cell memory and effector function. Cun. Opm. Immunol. 8: 331-36.

Lenschow D.J., Walunas T.L. Bluestone J.A. 1996. CD28/B7 system of T cell costimulation. Annu. Rev Immunol. 14: 233-58.

Parker D C. 1993. T cell-dependent B cell activation. Annu. Rev. Immunol. 11: 331-60.

Romagnani S. 1994. Cytokine production by human T cells in disease states. Annu. Rev. Immunol. 12: 227-57.