Carrier

Huw Davies, Division of Life Sciences, King's College, London, UK

Our current understanding of antibody production by B lymphocytes, and the underlying cooperation between T and B lymphocytes, owes much to the early use of hapten-carrier conjugates in immunologic studies. It had been known from the work of Landsteiner and of Pauling in the 1940s that haptens, small antigenic determinants such as dinitro-phenol (DNP), were insufficient by themselves to elicit specific antibody, but would bind to antibody once it was produced. The problem of how to generate hapten-specific antibodies was resolved by physical coupling of the hapten to a larger 'carrier' molecule. Proteins that have proved to be suitable carriers include bovine serum albumin, bovine gammaglobulin and ovalbumin.

We now understand that an efficient carrier is an immunogenic antigen that elicits a response from helper T lymphocytes. These cells will provide the necessary signals to enable B lymphocytes to generate hapten-specific antibodies. The following recapitulates the evidence for this. The first evidence that the carrier molecule itself was also recognized during an immune response to the hapten was provided by Ovary and Benacerraf in 1963. Animals primed with hapten A coupled to carrier B developed a secondary, anamnestic antihapten (and anticarrier) antibody response after boosting with the original A-B conjugate. However, no antibodies to A were seen if the mice were challenged with hapten A coupled to an unrelated carrier such as carrier C. This curious phenomenon was termed the 'carrier effect'.

The mid-1960s was an exciting period that saw the emergence of the small circulating lymphocyte, hitherto regarded as uninteresting, as a primary component of the immune system. This apparently homogeneous population of cells was soon split into two functionally different, but cooperative subsets. The pioneering work of Gowans, in which the immune system could be severely compromised by depletion of circulating lymphocytes, demonstrated the fundamental importance of this cell. A functional dichotomy was revealed by extirpation experiments by both Glick and Cooper, using chickens, and in mice by Miller. Their findings, that depletion of B lymphocytes by bursectomy abolished antibody production without affecting cell-mediated immunity, and that depletion of T lymphocytes by neonatal thymectomy ablated both antibody and cellular immune responses, laid the foundations of modern cellular immunology. A series of influential studies then fol-

lowed (Claman; Davies; Mitchell and Miller) which dealt with establishing that an antibody response required both T and B lymphocyte subsets. In essence, B cells which were shown to be responsible for the generation of antibodies, and T cells which helped by providing regulatory signals, both had to interact with each other before a specific antibody response could be elicited.

The next problem was to explain how the antigen fitted into this interaction between helper T cells and B cells. Hapten-carrier conjugates provided a powerful tool in addressing this question. It was already known that some proteins were better carriers than others. Immunization with hapten-carrier conjugates only induced hapten-specific antibody responses when the carrier to which the hapten was attached was itself 'immunogenic', that is if the carrier was a T cell (or thymus) dependent antigen. Nonimmuno-genic molecules served as poor carriers for haptens. This, and the 'carrier effect', indicated that separate recognition systems existed for the hapten and carrier, presumably mediated by B lymphocytes and T lymphocytes, respectively. The first direct evidence that cooperating T and B lymphocytes responded to different determinants on the same antigen molecule was provided by Mitchison (1971) using hapten-carrier conjugates. An adoptive transfer system, originally developed by Playfair and colleagues was used, in which lymphocytes were transferred from donor mice to a syngeneic recipient which had been rendered immunoincompetent by sublethal irradiation. T and B lymphocytes primed previously to different determinants were tested for their ability to cooperate within the recipient host (as manifested by the production of hapten-specific antibodies) in response to challenge with the determinants com-plexed on the same molecule. Mitchison observed that adoptively transferred spleen cells primed to hapten A conjugated to carrier B responded to challenge with A-B but not A-C (the carrier effect). However, if spleen cells from A-B primed mice were transferred together with cells primed to carrier C, an excellent response to challenge with A-C occurred. Moreover, hapten A and carrier C had to be physically linked.

These data, which have been confirmed in many experimental systems since, established that recognition of hapten and carrier is performed by different populations of cells that can cooperate to produce hapten-specific antibody, provided that the hapten and carrier are part of the same molecule. Formal proof that T lymphocytes recognized carrier determinants came from Raff in 1970. Employing the adoptive transfer system, depletion of Thy-11 lymphocytes (T cells) from the carrier-primed population using Thy-l-specific antibody plus complement abolished the adoptive antihapten response. Elimination of T lymphocytes from the hapten-primed population had no effect.

These experiments using defined B lymphocyte determinants coupled to carrier proteins revealed the importance of cognate T and B lymphocyte interactions in antibody synthesis and the recognition of spatially disparate determinants within the antigen by the cells participating in the response. A macro-molecular antigen can thus be regarded as a complex of carrier and haptenic determinants, or, more precisely, T helper and B lymphocyte epitopes.

Recent interest in carriers has been revived by the potential use of synthetic polypeptides which contain antigenic determinants for use as vaccines. The efficient generation of antibody responses to peptides usually requires the peptide to be coupled to an immunogenic carrier as a source of helper T cell determinants. In order that an appropriate anamnestic response is mounted upon subsequent pathogenic infection, the carrier must possess helper T cell determinants derived from the same pathogen as those recognized by the antibodies, thereby ensuring that T and B cells capable of cognate interaction in response to the pathogen will be present in the memory pool. Studies in inbred animals have shown that the helper T cell determinants above, in the form of synthetic peptides, can be used in place of the carrier molecule - thereby raising the prospect of vaccines comprising no more than arrays of contiguous T and B cell epitopes.

See also: CD40 and its ligand; B7 (CD80 & CD86); Cooperation, mechanisms of cellular; Cytokines; Epitopes; Hapten; Helper T lymphocytes; Vaccines

How To Bolster Your Immune System

How To Bolster Your Immune System

All Natural Immune Boosters Proven To Fight Infection, Disease And More. Discover A Natural, Safe Effective Way To Boost Your Immune System Using Ingredients From Your Kitchen Cupboard. The only common sense, no holds barred guide to hit the market today no gimmicks, no pills, just old fashioned common sense remedies to cure colds, influenza, viral infections and more.

Get My Free Audio Book


Post a comment