15 Instruction vs selection theory

Dr. M. N. Gupta

1. Objectives

•  To learn why and how instruction vs selection debate started

•  To learn how this debate was settled

•  To understand the basic difference between theories based upon instruction and selection

2. Concept Map

3. Description

In the very first module on history of immunology, we had briefly mentioned the long lasting controversies about the way antibodies with diverse specificities are created. Apart from its value as a historical perspective, a more detailed discussion about the basic philosophical difference between instructive theories and selection theories is useful. More techniques in biological sciences use the concept of selection than instruction.

The issue of instruction versus selection was crucial to the development of immunology. While chemists favoured instruction theories, biologists believed in selection. Understanding this debate is important to the subsequent discussion in the later modules. 

To fully appreciate this debate, it is important to remember the era, tools available and what we knew about protein structure and function in general.

Instruction versus Selection Theories

Instructionist models focussed on specificity of antibody and size of repertoire. The basic tenet was that the information for the antibody to become a functional entity required the presence of antigens. Hence, those are also called template models.

The main proponents were:

Breinl and Haurowitz 1930

Alexander 1932

Mudd 1932

Pauling 1940

The selection models were based upon the belief that antibody repertoire pre-existed and antigen picked one somehow.

The main proponents were:

Jerne 1955

Talmage 1957,1959

Burnet 1957

Lederberg 1959

Burnet 1959

The name of Burnet is repeated as in 1959 he outlined the clonal selection theory. He had also given an earlier version of the selection theory even earlier than Jerne. More on that later.

This long debate had centrestage in immunology and recounting it is useful to understand the thoughts which have shaped not only immunology but larger component of biological sciences.

Before delving into those chronological details, let us understand how these two approaches differ with the help of an analogy.

For buying a shirt we have two options:

One is to go to a tailor. Our presence is necessary. Instructions are given about our preferences by us: length, design, kind of fit etc. The tailor shop gives us back our „tailored‟ shirt. It did not exist before. Our presence was required to give instruction to tailor it to our specific needs. Think of the tailor shop as the immune system. Think of yourself as an antigen. Think of the shirt specifically made for us as the product of this response-the antibody.

This is in essence the concept of instructionist theories.

Let us choose another option. Let us go to a shopping centre or to your favourite mall. We go to a shop. Think of this as the encounter of yourself (antigen) with the immune system (the shop in this case). All the shirts pre-exist! You select the one specific to you or your taste.

This is essentially the essence of selection theories.

The difficulties were to visualize how these models explained:

Capacity to distinguish self vs non-self Presence of almost unlimited specific responses in terms of antibodies Memory Tolerance, that is, not only distinguish between self and non self antigens but tolerate self antigens and do not mount an immune response. Along with this each model had its own problem.

How could such large number of repertoire pre-exists? Think of a large number of foreigners from different countries visiting the dress shop along with you. Some of them are from countries where people tend to be tall, some from where people are relatively shorter, some where people tend to be lean! Please remember that the immune system responds to any unexpected antigen. Our shop will regret that they do not stock such large variety. They may offer to alter some shirts. Please remember this option. Later in this chapter we will see its biological equivalence.

During this long debate, a parallel debate also ensued. Ultimately, it was recognised that the immune response is just not limited to production/availability of a suitable.

specific antibody. The same questions have to be raised about T-cell mediated response.

Also, add to that there are various classes of antibodies (and there are subclasses!).

The challenge was to have a theory which explained it all. The debate also was influenced by the stature of the proponents. As we shall see, the debate was also shaped about how chemists and biologists tend to view science from their different perspectives. This is an exciting story. Let us go back to the beginning.

It is necessary to understand how new disciplines start. In the beginning, what we call immunology was actually the bacteriology. It started with Jenner and advanced considerably with Louis Pasteur. The concerns were what causes diseases and how to prevent them. Ironically, Louis Pasteur was not a medical person. Nor was Ellie Metchnikoff who was actually an embryologist. We have already referred to how Metchnikoff fiercely led scientists who thought all immunity is cell mediated. Nevertheless, these two kept the focus on pathogens and how body deals with them. Pasteur‟s work raised the hope that all pathogens responsible for infectious diseases could be identified and vaccines for all produced. By around 1910, it was realised that it is not so simple!

At this point, during the next 50 years, chemists became interested in how bodies fight diseases. Vaccinology morphed into a discipline which was more concerned with antibody-antigen relationship.

It was during this period that Karl Landsteiner appeared on the scene. He was awarded the Nobel Prize in 1930 for the discovery of blood groups which we know as part of ABO blood group system. Landsteiner‟s discovery of blood groups was a part of the larger set of studies. He can be considered among the top few immunochemist of his era.

An important discovery by Landsteiner was the discovery of the concept of „hapten‟: A small molecule which can trigger the immune response only when linked to a larger molecule. This included establishing that any compound arbitrarily picked from the shelf could provoke an immune response. So, immune response was not just about pathogens (and diseases). It was about antigenic compounds, antigenicity (size structure etc). It brought chemists into play. Before biologists, it was actually chemists who related structure with a property. We tend to forget that the lock and key hypothesis was given by Emil Fischer who was a carbohydrate chemist venturing into action of carbohydrases (glucosidases). Landsteiner carried out extensive work on the structure and function relationships of antibodies. That is, their specificities (and equally important cross specificities). If we remember 1940s, it was a powerful tool.

This was also a time when biologists were looking at genes and trying to understand how they are produced. This claim that immune system could make an antibody tailored to any compound (which need not have any biological relevance) was at variance with that emerging picture. Landsteiner‟s discovery meant that the biological system recognized the structures which were not present in nature.

Landsteiner‟s results gave support to template an instructive model. As cells were unlikely to store information which will enable it to make a tailored antibody not an infinite number of haptens was difficult to understand. That is the difficulty our shop had in having an infinite stock, Landsteiner‟s hapten concept also meant that shop had to also cater to a (small) pet accompanying a person!

In 1940, Linus Pauling published a paper called “A Theory of the structure and process of formation of antibodies” in Journal of American Chemical Society. Pauling has been one of the greatest chemists. He is the one who had explained what a chemical bond is and given the valence bond theory. That is what we are now taught in school chemistry.

All this is necessary to understand the impact of Pauling proposing a model for antibody formation.

What was known at that point?

“When an antigen is injected into an animal some of its molecules are captured and held in the region of antibody formation”

The number of binding sites on antibody was not known. Earlier, Marrack, Heidelberg and others had suggested that “precipitin reaction is a framework”, Pauling agreed with that. Haurowitz, Mudd and others suggested the idea of complementary structure for antibody and antigen. Ehrlich and Bordet implied it. So, Pauling wrote “The rule of parsimony (the use of minimum effort to achieve the result) suggests that there are only two such regions, that is, that the antibody molecules are at the most bivalent”

While Harowitz in 1930 and Mudd in 1932 had suggested that antibody molecule had a different “ordering of the amino acid residues”, Pauling differed and said that the difference is in the way “the chain is coiled in the molecule”

The point to note is that with so little to go on, Pauling was right about many fundamental aspects and so close to the real picture!

Pauling acknowledged that his “interest in immunology was awakened by conversations with Dr. Karl Landsteiner”.

Two years later, Pauling published another paper called “The manufacture of antibodies in vitro” using dyes like methyl blue and azophenylarsonic acid and mixing these with normal serum globulin, he showed that serum globulin developed affinity for these compounds.

These were taken as strong proof for the template theory or the instruction theory.

The compounds instructed the antibodies to be formed.

Before we go further, it is necessary to explain that Pauling ran these experiments carefully and with proper controls. What he did not know at that time was the non specific binding which proteins display. Based upon these non covalent interactions, macromolecules including proteins do show some “imprinting effects” under specific conditions. That Pauling observed these is curious.

The template theory remained unchallenged most of the time. Its acceptance also arose from the fact that it was difficult to understand how otherwise an organism could contain such large information which would be required if a selection theory were to work.

It may be added that there was another dimension of the debate. Was the origin of antibody diversity genetic in nature or was it somatic in nature? Selection theories would suggest that this was entirely genetic in nature. Template theory suggested that antibody specificity and diversity was purely somatic in nature.

What the template theory did not explain was the adaptive nature of the immune system. Immune system had a memory. Vaccinology initiated by Jenner and developed by Pasteur required it.

Understandably, the challenge to the template theory came from biologists.

Jerne’s Proposals

In 1995, Danish immunologist Niels Kaj Jerne recalled the earlier work (1949) of two Australians, F.MacFarlane Burnet and Frank J. Fenner.

Burnet and Fenner had noted the following phenomenon which template theory could not explain:

Exponential rise in antibody production during the early stages. How do the antibody outnumber the template?

Adaptive nature (as already explained) As immune reaction progresses, antibody fit antigen better (this phenomenon is known as affinity maturation) Most important was the phenomenon of tolerance Apart from referring to these concerns expressed by Burnet and Fenner, Jerne formulated a theory according to which diverse antibodies already existed. Once an antigen appeared on the scene, antigen-antibody complex was formed and it stimulated the white blood cells to produce the same antibody in larger amount.

At the University of Colorado, Denver; David W. Talmage saw the link between Jerne‟s theory and an earlier theory by Ehrlich. Ehrlich, decades back had proposed a side chain theory. According to that white blood cells had side chains to which foreign substances linked chemically. This resulted in the cell producing large number of copies of that side chain and excess side chains were shed in the blood. It is interesting to note that among other concerns, some scientists were worried about the chemical nature of the antigen binding to the side chains.

Talmage linked the Ehrlich‟s ideas with Jerne‟s proposals. In 1957, he suggested cells are selected for multiplication which match the invading antigen. He pointed out that as cancer cells produced homogeneous antibodies, cells can synthesize a specific antibody.

Burnet had said that antigen trigger self replication of cells. The nature of interaction with antigen was an issue. In later versions of his proposals, he felt that somehow antigen interacted with the genetic material of the cell.

It is to be noted that it is in view of the vagueness on some aspects, selection theories could not match the clarity of Pauling‟s proposals.

However, it is widely believed that Jerne and Talmage set Burnet on the right path. In his second generation of versions of his selection theory, he stated that antibody pre-exists. Binding of an antigen with the cell receptor triggered the production of the same receptor. Each cell or its clone (off springs) produced only one kind of receptor/side chain/antibody. This last feature was a leap of imagination. This also was the first time that “clonal selection” term was used.

Idiotypie Network Theory

The term „epitope‟ was coined by Jerne. This still persists but is largely replaced by “antigenic determinants”. According to Jerne an antigen has several epitopes. An antibody, apart from its own epitopes, has two “paratopes” (this term has fallen out of use now but one may come across it in old text books and literature). The paratopes were essentially antigen binding sites on the antibody.

Jerne clarified that “paratopes are not epitopes but anti-epitopes, keys are keys and locks are locks” (Jerne 1960). Jerne was referring to Fisher‟s metaphor for complementarity between enzymes and substrates.

Oudin along with Mauricette Michel in 1963 introduced the term “idiotypie” (in French) which in English was written as idiotypy.

Jerne and Janeway discussed Oudin‟s concept of idiotypy and Jerne defined idiotypes of an animal to be variable domains of the immunoglobulins present in that animal.

The antibodies directed against idiotypes are called idiotypic antibodies.

In 1969, Jerne visited Australia to attend Burnet‟s 70th birthday. The outcome was an article listing 10 things which were understood and 10 things which were not clear about antibody formation.

In the second list was:

“What controls the range of different antibodies that one animal or man can make?” (Jerne 1969). Jerne‟s idiotypie network theory (INT) thinks of the immune system as a network of mutually recognizing lymphocytes and antibodies. In this network, antibodies react with antibodies against them.

Jerne basically thought that clonal selection theory thinks of a lymphocyte becoming competent upon exposure to the antigen. Jerne‟s view was that the immune system functions independent of a foreign antigen.

According to INT, if the antibody was kind of a mirror image of an antigen (in terms of complementarity shape), the antibody2 against antibody, would be similar to antigen1.

Thus, an antigen may fit some combining site among this interacting network. Thus, foreign molecules, antigens merely modulate the network.

The set of combining sites which recognize the epitope also anyway was recognizing another set of pre-existing idiotopes in the network which can be thought of as internal image of the foreign epitope. Lymphocytes which display the internal image trigger duplication.

Jerne‟s theory tried to explain all other features of the immune system.

While Jerne‟s ideas received lot of support, the experimental data in support did not follow. Jerne was also awarded the Nobel “for theories concerning the specificity in development and control of the immune system.”

While clonal selection theory is now widely accepted, some of the ideas given by Jerne are considered relevant. There has been a renewed interest in idiotypic interactions in the clinical context. Much later, we will revisit that when we discuss autoimmune disease. Right now, in the next module, we will look at the clonal selection theory in detail.

Summary

We learnt about

• Ehrlich’s side chain theory

• Template theory

• Selection theories

• Idiotypic network theory

• Clonal selection theory