widespread fungal infections are unable to mount 

 an immune response because of overactivity of 

 suppressor T cells. If this suppressor function can 

 be regulated in such individuals, it may be possible 

 to cure or at least ameliorate their disease. This 

 strategy of suppressor T-cell regulation may also 

 be used to prevent graft rejection in patients with 

 kidney or other organ transplants. 



Research on the phenomenon of graft rejection 

 has resulted in identification of a number of "his- 

 tocompatibility locus antigens'" (HLA antigens) on 

 cell surfaces. NIAID-supported scientists across 

 the country — at Duke University, the University 

 of Wisconsin, the University of California, and 

 elsewhere — have demonstrated that matching 

 HLA antigens of donor and recipient in kidney 

 transplant operations greatly improves the chances 

 of the transplant being successful. Ninety percent 

 of kidneys transplanted between HLA-identical 

 siblings survive two years as compared to less than 

 50 percent two-year survival rates in grafts be- 

 tween HLA-mismatched siblings. 



Genes coding for these inherited HLA antigens 

 were mapped on chromosomes some years ago, 

 but it is only recently that this same chromosomal 

 region (known as the major histocompatibility lo- 

 cus) has been identified as the area in which Ir 

 (immune response) genes can also be found. One 

 of the first scientists to demonstrate genetic influ- 

 ence on the immune response — thus suggesting the 

 existence of such genes — was Dr. Baruj Benacer- 

 raf, an NIAID grantee at Harvard. Benacerraf's 

 work with guinea pigs has been repeated and ex- 

 tended by Dr. Hugh McDevitt, Stanford Universi- 

 ty, and others, including scientists in Dr. William 

 Paul's Laboratory of Immunology, NIAID. Their 

 work with animal models is also being successfully 

 applied to human studies and allergic disease. 

 Further delineation of the structure and function 

 of the products of the many linked genes of the 

 major histocompatibility locus is likely to provide 

 answers not only to the problem of genetic suscep- 

 tibility to disease but also to the mystery of organ 

 and tissue development during embryonic growth. 



Genetic Manipulation and DNA Recombination 



It has been known for some 25 or 30 years that 

 genes, the units of hereditary information, are 

 composed of a chainlike chemical called DNA 

 (deoxyribonucleic acid). It has been established for 

 about 20 years that hereditary information is en- 

 coded in the DNA molecule by the sequence of the 

 different types of "links" (nucleotide bases) that 

 make up the chain. This nucleotide sequence de- 

 termines if an organism will be a single-celled bac- 

 terium or a more complex multicellular plant or 

 animal, although our understanding of how this is 



accomplished is still very incomplete. In the past 

 five years, however, a powerful new methodology 

 has been developed by NIH grantees and other 

 researchers for probing into the mechanism by 

 which genes control the development and function- 

 ing of living cells. A variety of genes from virtually 

 any organism can be obtained in pure form, and 

 the new methodology of DNA recombination now 

 makes it possible to transfer genes of one kind of 

 organism to another and to construct genetic con- 

 stitutions that may never before have been seen in 

 nature. 



The new techniques have resulted from the dis- 

 covery of a class of enzymes, the "restriction 

 endonucleases," that can cleave a chain of DNA 

 into specific reproducible segments. Because of 

 the special way in which the chain is cut, the ends 

 are "sticky" and other enzymes (ligases) can inter- 

 changeably rejoin the pieces of DNA. By these 

 means, bits of DNA from different species can be 

 joined together, and reintroduced into bacterial 

 cells or into test tube cultures of cells derived from 

 tissues of higher organisms. The modified cells will 

 then grow and multiply, and pass copies of the 

 recombinant DNA on to new cells as they are 

 formed by cell division. 



A typical recombinant DNA experiment in- 

 volves the insertion of genes from a plant or ani- 

 mal into a laboratory strain of bacterium, Escheri- 

 chia coli K-12. This bacterium was established in 

 culture in 1922 and has been more extensively 

 studied than any other organism from a genetic 

 point of view. The genes to be inserted are joined 

 to plasmids, which are small extra strands of DNA 

 that sometimes occur in bacterial cells in addition 

 to their major chromosomes. Plasmids from E. coli 

 are extracted, purified, and modified by splicing on 

 new DNA segments derived from some other orga- 

 nism. Then they are reintroduced into living E. coli 

 K-12 cells. This process permits the functioning of 

 individual genes from higher organisms to be stud- 

 ied in the much simpler and better understood ge- 

 netic background of the bacterium. It also makes it 

 possible to do studies on isolated genes them- 

 selves, because it provides a convenient way to 

 obtain large quantities of purified genes through 

 "cloning," or growing identical cells in large num- 

 bers. Both approaches promise to enhance greatly 

 our understanding of how genes of higher orga- 

 nisms are expressed and regulated. 



Another area in which recombinant DNA tech- 

 nology offers hope of important progress is in 

 cloning modified cells for the large-scale produc- 

 tion of biological compounds for the treatment 

 and control of disease. For example, it is envi- 

 sioned that genes for insulin or other hormones 

 could be introduced into E. coli so that the bac- 



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