Genetic Mechanisms Involved in the Generation of the Antibody Repertoire 
gene are viable and appear normal at birth. How- 
ever, they routinely develop severe infections in 
the first months of life due to the total absence of 
mature T or B lymphocytes — a severe combined 
immune deficiency. These animals do have large 
populations of very immature B and T lympho- 
cytes in their primary lymphocyte differentiation 
organs; however, these cells are unable to initiate 
the VDJ recombination process. Because the 
.^G-2-deficient mice have no defects in any cell 
type besides lymphocytes, we have concluded 
that VDJ recombination activity and RAG- 2 gene 
function are required only for lymphocyte devel- 
opment and not for any other developmental 
process. 
Our group has also helped characterize the 
basis of a naturally occurring murine mutation 
that when homozygous leads to severe combined 
immune deficiency (the scid mutation). Like 
iL4G-2-deficient mice, animals homozygous for 
the scid mutation lack functional B or T lympho- 
cytes due to inability to assemble antigen recep- 
tor gene segments correctly. However, mice ho- 
mozygous for the scid mutation produce normal 
RAG-1 and -2 gene products and can efficiently 
initiate the VDJ recombination process. Comple- 
tion of the joining process is blocked in scid mu- 
tant cells. Moreover, both lymphoid and nonlym- 
phoid cells of mice homozygous for the scid 
defect are unable to repair certain types of lesions 
in their genetic material. Thus the product of 
the gene affected by the scid mutation appears to 
represent an example of a component of the 
VDJ recombination system that is used both in 
VDJ recombination and in more general cellular 
processes. 
To define further generally expressed genes 
potentially involved in VDJ recombination, we 
provided the lymphocyte-specific components of 
this system (RAG-l and -2) to mutant Chinese 
hamster ovary cell lines that are defective in abil- 
ity to repair breaks in their DNA. By introducing 
antigen receptor gene segments into these lines, 
we could ask whether the cells were capable of 
performing correct VDJ recombination. By this 
approach, we have now derived a series of inde- 
pendent genetic mutations that affect both DNA 
repair and VDJ recombination. Introduction of 
specific human chromosomes into the mutant 
cell lines restored both ability to repair DNA and 
to undergo VDJ recombination. We are now in the 
process of isolating the defective genes in these 
cells. Elucidation of these genes will provide in- 
sight into the VDJ recombination process and may 
also provide information relevant to understand- 
ing the basis for several human diseases that affect 
both ability to repair genetic lesions and to gener- 
ate a normal immune system. 
We are also analyzing several other novel 
mouse models generated to study various factors 
involved in lymphocyte differentiation and the 
generation of the immune response. One such 
model is a mutant mouse line that cannot pro- 
duce endogenous antibody molecules because it 
has been genetically engineered to lack germline 
gene segments necessary for forming functional 
antibody genes. We are using these animals to 
study the role of antibody gene products in regu- 
lating antibody gene assembly and lymphocyte 
development. A potential practical use of such 
mice may be achieved by breeding them with 
transgenic mouse lines that have been engineered 
to contain functional human antibody gene cas- 
settes. The hope is that the hybrid animal will 
rely on the human antibody genes for its immune 
system, providing a more effective method of 
generating tailor-made human antibodies. 
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