Overview of the Induction of a Humoral Immune Response to an Immunogen

OVERVIEW OF THE INDUCTION OF A HUMORAL IMMUNE RESPONSE TO AN IMMUNOGEN

A. Exposure to Natural Immunogens

Infectious agents penetrate the organism via the skin, upper respiratory mucosa, and intestinal mucosa. In most cases the immune system is stimulated in the absence of clinical symptoms suggestive of infection (subclinical infection). The constant exposure to im-munogenic materials penetrating the organism through those routes is responsible for con-tinuous stimulation of the immune system and explains why relatively large concentrations of immunoglobulins can be measured in the serum of normal animals. In contrast, animals reared in germ-free conditions synthesize very limited amounts of antibodies, and their sera have very low immunoglobulin concentrations.

B. Deliberate Immunization

Many infectious diseases can be prevented through active immunization. When live atten-uated organisms are used for immunization, they are usually delivered to the natural portal of entry of the organism. For example, a vaccine against the common cold using attenuated rhinoviruses would be most effective if applied as a nasal aerosol. On the other hand, if in-ert compounds (such as inactivated infectious agents, polysaccharides, or toxoids) are used as immunogens, they have to be introduced in the organism by injection, usually intramus-cularly, subcutaneously, or intradermally.

In humans, immunization is usually carried out by injecting the antigen intrader-mally, subcutaneously, or intramuscularly or by administering it by the oral route (e.g., at-tenuated viruses, such as poliovirus). As a rule, injected immunogens are mixed or emulsi-fied with adjuvants, compounds that enhance the immune response.

The most potent adjuvant is complete Freund’s adjuvant (CFA), a water-in-oil emul-sion containing killed mycobacteria. This adjuvant is widely used for the production of an-tisera in laboratory animals, but it tends to cause an intense inflammatory reaction in hu-mans and for that reason is not used in human vaccines. In humans, the most commonly used adjuvants are inorganic gels such as alum and aluminum hydroxide.

The mechanism of action of all adjuvants is similar and involves two important fac- tors:

1. Adjuvants slow down the diffusion of the immunogen from the injected spot, so that antigenic stimulation will persist over a longer period of time.

2. Adjuvants induce a state of activation of antigen-presenting cells in the site of in-oculation. This activation can be more or less specific: CFA causes a very intense local inflammation, while some bacterial compounds with adjuvant properties have a more targeted effect on macrophages and other APC and have strong ad-juvant properties without causing a very intense inflammatory reaction. Alum and aluminum hydroxide share both types of effects, but they do not induce an inflammatory reaction as intense as CFA, and this is the reason for their use in human immunization.

C. B-Cell Activation

B-cell activation requires multiple sig-nals. The only specific signal is the one provided by the interaction between the antigen-binding site of membrane immunoglobulins with a given epitope of an immunogen. The additional signals, provided by TH2 helper cells and APC, are nonspecific with regards to the antigen.

The recognition of an antigen by a resting B cell seems to be optimal when the im-munogen is adsorbed to a follicular dendritic cell or to a macrophage. B lymphocytes rec-ognize either unprocessed antigen or antigen fragments that conserve the configuration of the native antigen. All techniques used for measurement of specific antibodies use antigens in their original configuration as their basis and succeed in detecting antibodies reacting with them. Whether or not some B cells may have membrane immunoglobulins reactive with immunogen-derived peptides associated with MHC-II molecules is not known.

Helper TH2 lymphocytes provide other signals essential for B-cell proliferation and differentiation. The activation of this CD4 subpopulation is favored by a low-affinity in-teraction between an immunogen-derived oligopeptide associated with an MHC-II molecule and the T-cell receptor, as well as by additional signals, some derived from cell-cell interactions, such as the CD28/CD86 interaction, and others from cytokines, such as IL-4 and IL-1. Activated TH2 cells, in turn, provide several co-stimulatory signals that pro-mote B-cell proliferation and differentiation. Some of these signals derive from the inter-action between cell membrane molecules upregulated during the early stages of TH2 and B-cell activation, e.g., CD40L (CD159) on T cells and CD40 on B cells, while others are mediated by cytokines, such as IL-4, IL-6, and IL-10.

When the proper sum of specific signals and co-stimulatory signals is received by the B cell, clonal proliferation and differentiation ensues. Since each immunogen presents a multitude of epitopes, a normal immune response is polyclonal, i.e., it involves many dif-ferent clones recognizing different epitopes of an immunogen. The induction of an immune response requires some time for activation of all the relevant cells and for proliferation and differentiation of B cells into plasma cells. Thus, there is always a lag phase between the time of immunization and the time when antibodies become detectable. It must be noted that while most activated B cells will become antibody-producing plasma cells, a few will become memory cells .

Experimental animals immunized with a given immunogen (e.g., tobacco mosaic virus) often show marked postimmunization hypergammaglobulinemia, but only a very small fraction of the circulating immunoglobulins reacts with the immunogen. In humans, the initial burst of IgE production, after first exposure to an allergen, seems to be mainly constituted by nonspecific antibodies. This apparent lack of specificity of the immune re-sponse is more obvious after the first exposure to an immunogen. Most likely is a conse-quence of the fact that the antibodies produced early in the immune response are of low affinity and may not be detectable in assays that favor the detection of high-affinity anti-bodies. It is also possible that the co-stimulatory signals provided by TH2 cells may en-hance the immune response of neighboring B cells engaged in unrelated immune responses. This may result in the enhanced synthesis of antibodies reacting with other immunogens that the immune system is simultaneously recognizing. While the synthesis of unrelated an-tibodies is usually beneficial or inconsequential , it can also be the basis for at least some autoimmune reactions if strong help is provided to autoreactive B cells, which otherwise would remain quiescent