Polyclonal B cell response

Blind Monks Examining an Elephant: An allegory for the polyclonal response: Each clone or antibody recognizes different parts of a single, larger antigen.

Polyclonal response is a natural mode of immunological response exhibited by the adaptive immune system. It ensures that a single antigen is recognized and attacked through its multiple overlapping parts, called epitopes, by multiple clones of B lymphocytes.

In course of a normal immune response, a foreign substance present on a pathogen such as an invading virus, is recognized by the body. The immune system reacts against the substance to eliminate it or to reduce the damage the pathogen causes. Such a recognizable foreign substance is known as an antigen. The immune system may respond in multiple ways to an antigen; a very important component of this response is the production of antibodies by the B cells (or B lymphocytes), which are a type of white blood cell. This arm of the adaptive immune system is known as humoral immunity, as the antibodies are soluble and do not require direct cell-to-cell contact to destroy the pathogen, unlike cell-mediated immunity.

Antigens can be large and complex substances, and any single antibody can only bind to a small, specific area. Consequently, an effective immune response often involves the production of many different antibodies by many different B cells. Together, the response by these B cellsis called a polyclonal immune response.

B cell response

An antigen is any substance (usually a protein) that can be 'recognized' by the host organism. On a molecular scale, the proteins are relatively large, so they cannot be recognized as a whole; instead, their segments, called epitopes, can be recognized.

Figure 1: Schematic diagram to explain mechanisms of clonal selection of B cell, and how secondary immune response is stronger, quicker and more specific in comparison with the primary one.

Antigen presentation

When an antigen presenting cell (APC) like the macrophage or the B lymphocyte engulfs an antigen through phagocytosis, its lysosomes break the antigen down into various peptides. The individual peptides are then complexed (attached loosely) with major histocompatibility complex class II (MHC class II) molecules located in the lysosome--this method of "handling" the antigen is known as the exogenous pathway of antigen processing in contrast to the endogenous pathway, which complexes the proteins produced within the cell (say under the influence of a viral infection or in a tumor cell) with MHC class I molecules. From here the complex migrates to the plasma membrane, and is exhibited there (elaborated) as a complex that can be recognized by the CD 4+ (T helper cells). This is known as antigen presentation. Note, however, that the epitopes (conformational epitopes) that are recognized by the B cell prior to their digestion may not be present on the peptides presented with the MHC class II molecules. Major histocompatibility complex is a region on the DNA containing genes present in all the nucleated cells of all jawed vertebrates, that codes for many products involved in antigen presentation (class I and II MHC molecules) and functioning of the complement system. The complement system is an important additional tool of the body in directly attacking the individual microorganisms. The MHC molecules in humans are also known as human leukocyte antigen.

The antigen-receptor (epitope-paratope) interaction

Whatever the cell type, recognizing an antigen or a segment thereof (an epitope) requires the antigen to bind with the corresponding paratope that is present on the receptor, which is in turn present on the surface of the recognizing cell. In the immune system, these are the T cell receptor (TCR) and the B cell receptor (BCR). The binding between a paratope and its corresponding antigen is very specific owing to their structures and is guided by various noncovalent bonds not unlike pairing of other types of ligands (any atom, ion or molecule that binds with any receptor with at least some degree of specificity and strength) their corresponding receptors.

T helper cell stimulation and B cell costimulation

The CD 4+ cells through their TCR recognize the epitope-bound MHC II molecules on the surface of the APCs, and get 'activated'. However, complete stimulation requires the B7 molecule present on the APC to bind with CD28 molecule present on the T cell surface close to the TCR. When this activated T cell encounters a B cell that recognizes the antigen containing the same epitope as recognized by TCR, the latter (B cell) gets stimulated because of secretion of certain growth factors, viz., interleukins 2, 4, 5, and 6 in a paracrine fashion. However, this activation occurs only when the BCR present on a memory or a naive B cell itself is bound to the corresponding epitope. A naive (or inexperienced) B cell is one which belongs to a clone which has never encountered the epitope to which it is specific. In contrast, a memory B cell is one, which derives from an activated naive or memory B cell. This is followed by a manifold proliferation of that particular B lymphocyte, most of the progenies of which terminally differentiate into plasma cells, which secrete the antibodies (first immunoglobulin M, and then immunoglobulin G, in that sequence) that bind to the same epitope structure that had bound stimulated the B cell in the first place by binding to its BCR. The rest survive as memory B cells. So, when in the "history of a clone", naive cells encounter their specific antigen to give rise to the plasma cells (that neutralize the same antigen by binding it), and also leave a few memory cells, this is known as the primary immune response. In the course of this proliferation, the BCR genes can undergo somatic hypermutation (frequent {1 in every two cell divisions} mutations in the genes coding for paratopes of various receptors), making the B cell receptors and the soluble antibodies in subsequent encounters with antigens, more inclusive in their antigen recognition potential, as well as more specific for the antigen that induced proliferation in the first place. Enhanced affinity that follows multiple rounds of proliferation and hypermutation is known as affinity maturation. When the memory cells get stimulated by the antigen to produce plasma cells (just like in the primary response), and leave even more memory cells in the process, this is known as a secondary immune response, which along with the soluble antibodies that plasma cells of the clone produce, also recognize the corresponding antigen faster (owing to somatic hypermutation) and bind more strongly with them (avidity of binding). Additionally, more memory cells translates into greater number of plasma cells. This results in higher antibody concentrations being reached in blood for longer periods.

Basis of polyclonal response

Figure 2: Schematic diagram showing polyclonal response by B cells against linear epitopes

Polyclonal derives from the words poly, meaning many, and clones. A clone is a group of cells with common ancestry ("mother" cell).

Clonality of B cells

Memory and naïve B cells normally exist in relatively small numbers. As the body needs to be able to respond to a large number of potential pathogens, it maintains a diverse repertoire of B cells. Consequently, while there is almost always at least one B (naive or memory) cell capable of responding to any given pathogen, there are very few exact duplicates. However, when a single B cell encounters an antigen to which it can bind, it can proliferate very rapidly, with the number of reactive cells doubling every 90 minutes. Each such group of cells with identical specificity towards the epitope is known as a clone, and is derived from a common "mother" cell. All of the "daughter" B cells match the original "mother" cell and secrete antibodies with identical paratopes.

Single antigen contains multiple overlapping epitopes

A single antigen can be broken down into multiple overlapping epitopes (see Figure 1). Many unique B cells may be able to bind to these different epitopes. This imparts even greater multiplicity to the overall response. All of these B cells can become activated and produce large colonies of plasma cell clones, each of which can secrete upto 2000 antibody molecules against each epitope per second.

Multiple clones recognize single epitope

In addition to different B cells reacting to different epitopes on the same antigen, B cells belonging to different clones may also be able to react to the same epitope. An epitope that can be attacked by many different B cells is said to be highly immunogenic. In these cases, the binding affinities for respective epitope-paratope pairs vary, with some B cell clones producing antibodies that bind strongly to the epitope, and others producing antibodies that bind weakly. This binding requires both the paratope and the epitope to undergo slight conformational changes in each others' presence.

Clonal selection

The clones that bind to a particular epitope with sufficient strength are selected for further proliferation in the germinal centers of the follicles in various lymphoid tissues like the lymph nodes. This is not very different from Darwinian concept of natural selection: clones are selected for their fitness to attack the epitopes on the current pathogen.

Diversity of B cell clones

In spite of the fact that there are so many diverse pathogens, many of which constantly keep on mutating, it is a great surprise that a majority of individuals remain infection-free. This maintenance of disease-free state requires the body to recognize as many pathogens (antigens they present or produce) as known to exist. This is achieved by maintaining a pool of immensely large (about 10⁹) clones of B cells, each of which reacts against a specific antigenic determinant, recognizing it and producing antibodies against it. Thus, approximately 10⁷ different epitopes can be recognized by the all the B cell clones combined.. However, at any given time very few clones actually remain receptive to their specific epitope. Moreover, in a lifetime, an individual usually requires to generate antibodies against very few antigens in comparison with the number that the body can recognize and respond against.

Epitope recognition by B cell

Figure 3: Recognition of conformational epitopes by B cells. Note how the segments widely separated in the primary structure have come in contact in the three dimensional tertiary structure forming part of the same epitope

The tertiary structure of a protein is like a woolen ball with complex loops and folds, in which certain segments would be visible, and others obscured by those lying superficially. Whereas, the primary structure is like a stretched out strand of wool.

In Figure 1, the various segments that form the epitope have been shown to be continuously collinear, meaning that they have been shown as sequential, however, for the situation being discussed here, i.e., antigen recognition by the B cell, this explanation would prove to be too simplistic. These are known as linear or sequential epitopes as all the amino acids on them are in the same sequence (line). This mode of recognition is possible only when the peptide in question would be small (to the order of six to eight amino acids long), and is employed by the T cells (T lymphocytes).

However, the B memory/naive cells recognize intact (meaning, undigested, and not that the whole protein structure is recognized at the same time) proteins present on the pathogen surface. In this situation, the proteins in their tertiary (the three dimensional structure as against the linear or primary structure) structure are so much folded that it is very unlikely that all the continuous segments of the protein will lie close to each other in space while interacting with the receptor. So, the paratope on the BCR in these cases actually recognizes the discontinuous segments of proteins that would have come close to each other owing to complex folding patterns of the protein (see adjoining Figure 2). Such epitopes are known as conformational epitopes and tend to be longer in length (15-22 amino acid residues) than the linear epitopes. Likewise, the antibodies produced by the same plasma cells belonging to the same clone would bind to the same conformational epitopes located on the pathogen proteins.

In the above analogy of a whorl of wool, if it would be possible to cut out a chunk of it that would correspond to a conformational epitope which upon unfolding will give many short segments. And, cutting a strand into short segments would give us a linear epitope.

Increased probability of recognizing any antigen

If an antigen can be recognized by more than one components of its structure, it is less likely to be "missed" by the immune system. Analogically, if you are in a crowded place, one is supposed to recognize a person; it is better to know as many physical features as possible. If you know the person only by the hairstyle, there is a chance of overlooking the person if that changes. Whereas, if apart from the hairstyle, if you also happen to know the facial features and what the person will wear on a particular day, it becomes much more unlikely that you will miss that person. Here the concept of mutation of pathogenic organisms is being explained, which can result in modification of antigen (and, hence, epitope-) structure. Now, if the immune system "remembers" what the other epitopes look like, the antigen, and the organism will still be recognized and subjected to body's immune response.

Limitation of immune system against rapidly mutating viruses

Figure 4: A high affinity memory B cell, specific for Virus A, is preferentially activated by a new strain, Virus A¹, to produce antibodies that ineffectively bind to the A¹ strain. The presence of these antibodies inhibits activation of a naive B cell that produces more effective antibodies against Virus A¹. This effect heightens the potential for serious infection.

Many viruses have enzymes (polymerases) defective in proofreading of their genetic material during replication. This allows certain changes in amino acid composition of their important proteins (mutations). When these proteins can perform their assigned functions (generally binding to some host protein) even in the face of these mutations, the B memory cell(s) that would have recognized the protein in prior encounter still recognize the protein (antigen), but the antibodies that they produce upon proliferation do not bind with the antigen sufficiently strongly. This is unfortunate because somatic hypermutation does give rise to clone capable of producing soluble antibodies that would have bound the pathogen strongly enough to neutralize it. But these clones would consist of naive cells, which are not allowed to proliferate by the weakly binding antibodies produced by the priorly stimulated clone. This doctrine is known as the original antigenic sin.

This phenomenon comes into play particularly in immune responses against influenza, dengue and HIV viruses.

This limitation, however, is not imposed by the phenomenon of polyclonal response, but rather, against it by an immune response that is biased in favor of experienced memory cells against the "novice" naive cells.

Increased chances of autoimmune reactions

The phenomenon of autoimmunity can be most simply explained in terms of the immune system making mistake by wrongly recognizing certain native molecules in the body as foreign, and in turn mounting an immune response against them. Since these native molecules, as normal parts of the body, will naturally always exist in the body, the attacks against them can get stronger over time. Moreover, many organisms exhibit molecular mimicry, which involves showing those antigens on their surface that are antigenically similar to the host proteins. This has two possible consequences: first, either the organism will be spared, as a self antigen, or secondly, that the antibodies produced against it will also bind to the proteins that the organism would have mimicked, and the harboring tissue will come under attack by various mechanisms like the complement activation and Antibody-dependent cell-mediated cytotoxicity. Hence, if the body produces more varieties (differing specificities as a result of polyclonal response) of the antibodies, greater the chance that one or the other will react against self-antigens (native molecules of the body).

Production of monoclonal antibodies

Monoclonal antibodies are structurally identical immunoglobulin molecules with identical epitope-specificity (all of them bind with the same epitope with same strength {avidity}) as against their polyclonal counterparts which have varying affinities for the same epitope. Monoclonal antibodies find use in various diagnostic modalities (see: western blot and immunofluorescence) and therapies—particularly of cancer and diseases with autoimmune component. But, since virtually all responses in nature are polyclonal, it makes production of immensely useful monoclonal antibodies less straightforward.

See also

• Polyclonal antibodies
• MHC class II
• Antigen processing
• Antiserum, a polyclonal antibody preparation used to treat envenomation
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References

• Goldsby, Richard. Immunology (Fifth Edition ed.). New York: W. H. Freeman and Company. ISBN 07167-4947-5. {{cite book}}: |edition= has extra text (help); Unknown parameter |coauthors= ignored (|author= suggested) (help)