Immunology Today

January 1983

I r g.enes

New tests of familiar arguments fromJ. Kapp, C. Pierce, S. Cullen, D. Shreffler and B. Schwartz

T-cell- specific markers associated with the I-A and 1-C subregions and low molecular weight molecules encoded by a gene or genes in the K-1-A interval were described. Thus, it appears that the number of / region genes and products of

TJ aditionally, definition of the multiple genes of the I region has focused on the se~ ological analysis of intra-H-2 recom- binant mice and the pattern of immune responses developed by these recom- binant mice. Since the last Ir Gene Workshop in 1978, application of several new techniques, including gene cloning and DNA sequencing, production of T- and B-cellhybridomas, and development of cloned lines of T and B cells, has introduced a new phase into the analysis of I t genes*.

Many serological and biochemical studies have suggested that Ia antigens are more heterogeneous than had been previously appreciated. A number of laboratories reported studies with mono- donal antibodies which indicate that within a single strain, multiple discrete a- and/J-chains exist within the 1-/i and I-E subregion antigens. Some, but not all, of this heterogeneity could be due to intra- subregion a//J combinations. Evidence was also presented that at least two and probably three human Ia molecules exist. Some antigenic heterogeneity is ap- parently due to heterogeneity of carbo- hydrate groups as shown by the detailed oligosaccharide side-chain structure of Ia molecules reported by S. Cullen (St Louis). It is of particular note that many different complex oligosaccharide struc- tures are associated with the same Ia molecule, which suggests that carbo- hydrates have a possible functional role. Also, the I-B subregion refuses to go away; published functional studies to explain the IgG allotype response in terms of complementation have been proposed but new genetic analyses ap- pear to favor the existence of the I-B subregion. Many laboratories have shown the existence of two or more epitopes per Ia molecule which are

*The Fifth Ir Gene Workshop was held in St Louis on 28-31 August.

recognized by different monoclonal anti- bodies. Initial studies of anti-idiotype antibodies to monoclonal anti-Ia anti- bodies offer promise for probing the nature of the T-cell Ia receptor. The inter-species relationships of Ia mole- cules are also being defined.

Determinants encoded by the I - ] sub- region have been shown on a number of functionally different subclasses of sup- pressor T cells, T-cell-derived factors, and on a subclass of adherent cells. The question of how many products are en- coded by/- j r subregion genes was raised and cross-absorption studies suggested that more than one exists. The develop- ment of many new monoclonal anti-I-J reagents and suppressor T-cell hybrid- omas of specific functional subsets is in progress and should lead to definitive biochemical answers to this question in the near future.

these genes is continuing to expand. However, the invariant chain which is associated with Ia antigens in the cytosol clearly has been shown to be encoded by a gene which is not linked to H-2.

Gene s t ructure For the first time in an Ir Gene

Workshop, data on the structure of genes encoding Ia antigens were presented. H u m a n DRat and mouse 1-Ea and I-A/J genes have two exons encoding the two major domains of the chains, but differ in the detail of the exons determining the transmembrane and intra-cytoplasmic portions of these proteins. Amino acid sequences derived from DNA sequences indicated that D R a and LEa chain had one internal disulfide bond and poten- tially two N-linked oligosaccharide side

continued on p. 3

I m m u n o t o x i c i t y .

Undesirable effects of inappropriate responses

from G. E. Davies

Immunology appears to be eminendy suitable for fusion with other sciences: hybrids with chemistry, pathology, genetics and pharmacology have been with us for some time. The latest addition to this growing family is immunotoxi- cology. *

A major reason for the current interest in the subject is the anxiety felt in industry that mandatory testing for immunotoxic effects may be imminent. These fears, though, were to some degree set at rest by representatives of regulatory bodies such as the Department of Health and W H O • An international symposium on immunotoxi- cology was held at the University of Surrey at Guildford on 13-17 September.

who offered the assurance that imposed : tes t~g is some way off.

Only one attempt was made to define ' immunotoxicity' - and that was by the present writer. I suggested that it is ' the undesirable effects of an inappropriate response of the immune system'. 'Effect' was distinguished from 'response' because, for example, inhibition of anti- body production is not, in itself, un- toward but its effect may be. The term 'inappropriate ' was used to indicate that a given response may be either desirable or undesirable, according to the circum-

s t ance s (depression of an immune response may facilitate graft retention but it might also facilitate infection). It is

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this Janus attitude which represents one of the several dilemmas facing the im- munotoxicologist. The question 'why immunotoxicology?' may be asked with some cogency. There are two answers: the reasoned, theoretical possibility that a disturbance of immune mechanisms might lead to a greater incidence of infec- tion or neoplasia; and the practical possi- bility that immunotoxic effects have been seen in man. The most completely docu- mented example of the latter was dis- cussed by J . G. Bekesi (Mount Sinai School of Medicine, New York), namely the consequences of accidental exposure of 500 dairy farmers in Michigan to poly- brominated biphenyls. The observed ab- normalities included hypergammaglob- ulinaemia, a significant increase in the absolute number and percentage of T and B lymphocytes, an increase in the number of lymphocytes without surface markers and altered lymphocyte func- tion. So far, though, no further serious consequence has been observed. Other reported examples of immunotoxic effects in man (apart from the obvious undesirable side-effects of immunosup- pressive therapy) are rare, especially those exemplifying specific immuno- toxicity and not disturbance of immune function consequent on systemic non- specific toxicity: this is not to say that such reactions do not occur, but simply that unequivocal, confirmed evidence of their existence is hard to find.

Models of immunotoxicity in animals were discussed by (among others) two pioneers of the subject: J . H. Dean (Chemical Industry Institute of Toxi- cology) and J . G. Vos (Rijks Instituut Voor de Volksgezonheld). Both are pro- ponents of a 'tiered' approach to these studies, Tier 1 comisting of simple and inexpensive observations (cost- effectiveness being important) and Tier 2 consisting of a more complex battery of assays designed to identify the affected parts of the immune system. Functional tests, involving experimental infection with bacteria and viruses, and studies with spontaneous and induced tumours

and autoimmune conditions are current- ly receiving attention and appear to have much to commend them. Animal models ofimmunotoxicity, especially if intended to be predictive of effects in man, are beset by two major problems: repro- ducibility and relevance. Such problems of course are not confined to immuno- problem, whilst at the same time attract- ing 'model collectors': this is worrying. The recent expansion inknowledge of the immune system has led to the availability of an exceedingly large number of tech- niques for studying the effects of chemi- cals on the immune system both in vivo and in vitro, and for immunopharma- cological studies this is commendable. However, use of such a multiplicity of models, in different laboratories, as tests to determine the potential safety of new drugs and chemicals, can lead to chaos. The time is surely ripe for the leading experts in the field to get together to reach a consensus.

Hypersensitivity reactions were quite properly included in this symposium on immunotoxicity. There is less un- certainty about the reality of the problem than appears to be the case for the im- munotoxic effects considered above. All four types of hypersensitivity classified by Gell and Coombs are seen as allergic reactions to haptens in man and there is some evidence that some granulomatous reactions may form a fifth type: such reactions were described by J. Turk (Royal College of Surgeons) and exemp- lifted by A. L. Reeves (Wayne State University) in a discussion on the immunotoxicity of beryllium. With the possible exception of tests for contact sensitization the predictive value of animal models for potential allergenicity ofhaptens remains unsatisfactory. It was generally agreed that there is no suitable predictive model of hypersensitivity of types 1-3. In fact it was reported by J. Doe (ICI) that inhalation of toluene diiso- cyanate by guinea pigs induced a state of tolerance rather than hypersensitivity.

The role of metabolism, particularly that dependent on the mixed function

Immunology Today, vol. 4, No. 1, 1983

toxicity; species, sex and strain dif- ferences in susceptibility bedevil all toxicity tests as do seasonal and circadian rhythms, dietary and environmental in- fluences, etc. The enormous complexity of the immune system magnifies the oxidase system was discussed in detail by D. 'V. Parke (University of Surrey) but there seemed to be no general agreement that conjugates formed in vivo by this mechanism were, in fact, immunogenic. There is, though, a 'Catch 22' here because antibodies to an uncharacterized metabolite can be detected with certainty only by the use of the same uncharacter- ized metabolite.

One of the major problems confront- ing the pharmaceutical industry is the rare, idiosyncratic reaction. M. Davis (Dudley Road Hospital, Birmingham, U.K.) reported that massive hepatic necrosis after exposure to halothane follows about 1 in 20 000 to 1 in 30 000 administrations, and it is difficult to see how such a low incidence can ever be 'modelled'. The same speaker described experiments in rabbits which showed that hepatocytes from animals anaesthetized with halothane became susceptible to immune attacks by lymphocytes from patients with halothane-associated hepa- titis. The 'missing-link', however, re- mains missing - why, in man, is the incidence so low?

' Pseudo-immunotoxicity' received attention from W. Lorenz (Phillips- Universit~it Marburg) in his discussion of the hypersensitivity reactions induced by intravenous anaesthetics and plasma substitutes reactions that should probably be studied more appropriately in pharmacology than immunology.

Immunotoxicology, however, is a legitimate and topical field of study. We will hear a good deal more about it in 1983.

G. E. Davies is Head of Immunology at the Central Toxicology Laboratory, Imperial Chemical Industries PLG, Alderley Park, Nr Macclesfield, Cheshire SKlO 4TJ, U.K.