regulated genes, showing overlap between IFN-7 
and IFN-a gene regulation. These mutants, which 
fall into one complementation group, are defective 
in an early event during activation of the transcrip- 
tion factors mediating IFN-a: and IFN-7 effects. The 
group of Dr. James Darnell (Rockefeller University) 
has recently shown that a transcription factor that 
mediates IFN-a activation (ISGF3-a) also contains a 
subunit that is activated by IFN-7 (GAF) . In collabo- 
ration with Dr. Darnell, Dr. Flavell's group has 
shown that these mutants fail to phosphorylate both 
the 7 and a subunits, upon stimulation of the recep- 
tors. These polypeptides seem nonetheless to be 
present within the cells, pinpointing the likely de- 
fect at an early stage of phosphorylation of these 
proteins prior to nuclear translocation. The mutants 
also fail to translocate these factors to the nucleus. 
The second major class of mutants cannot transac- 
tivate class II MHC and the gene for the class II- 
associated polypeptide in the invariant chain. The 
first three mutants analyzed in this group also con- 
sist of a single complementation group. cDNA ex- 
pression libraries are being used to complement 
these defective mutants in order to clone the gene 
responsible for the defect. 
The invariant chain is believed to play a role in the 
association of antigenic peptides with MHC. It prob- 
ably targets MHC class II to endosomes where anti- 
gen is bound and the invariant chain dissociated. To 
elucidate the function of invariant chain in vivo. Dr. 
Flavell's laboratory has generated "knock-out" mice 
defective in their endogenous invariant chain genes, 
by the use of homologous recombination. These 
mice are under analysis to identify their immune 
defects and elucidate further the role of the invari- 
ant chain. 
Lineage Relationships and Biologic Role 
of CD4 and CDS T Cell Subsets 
Although the development of T cells in the thy- 
mus is becoming clear, T cell differentiation in the 
periphery is poorly understood. CD4 T cells fall into 
two classes expressing different cytokines. THl 
cells produce IFN-7, IL-2, and TNFs, whereas TH2 
cells produce IL-4, IL-5, and IL-6. The lineage rela- 
tionships between these cells and precursors that 
exit the thymus are not understood. To elucidate 
this and to obtain a better understanding of their 
functional role. Dr. Flavell's laboratory, in collabora- 
tion with Dr. Kim Bottomly (HHMI, Yale Univer- 
sity), has developed a transgenic model whereby 
such cells can be specifically ablated. The IL-2 and 
IL-4 promoters have been used to direct the synthe- 
sis of HSV-TK (herpes simplex virus thymidine ki- 
nase) to the subsets that produce these cytokines. 
Upon activation of these genes, the cells proliferate 
and can be killed by ganciclovir; this approach was 
developed originally for other systems by Dr. Ron- 
ald Evans (HHMI, Salk Institute for Biological Stud- 
ies). Transgenic mice utilizing the IL-2 promoter 
produce T cells that are readily susceptible to kill- 
ing by ganciclovir in vitro. The validation of this 
system in a series of standardization experiments has 
shown that it works and is capable of addressing the 
questions posed. Current activities, therefore, are 
directed at elucidating the precise relationship of 
IL-2-, IL-4-, and IFN-7-producing CD4 T cell 
subsets. 
Dr. Flavell's laboratory has used these mice to ad- 
dress the differentiation of CDS T cells. It is known 
that some CDS T cells produce IL-2 (CDS helper 
cells) , whereas others develop effector function and 
become CTLs. The relationship between CDS 
helper cells and CTLs is not clear. This laboratory 
has utilized T cells from TCR transgenic mice, dou- 
bly transgenic for the IL-2-TK (TK, thymidine ki- 
nase) construct, to answer this question. Growth in 
vitro of such cells, in the presence of antigen and 
ganciclovir, eliminates their proliferative response 
and the production of CTLs. Since the CTLs are not 
restored by the addition of IL-2, this experiment 
suggests that the so-called CDS helper cells are pre- 
cursors of the CTLs or even the same cell. 
Protective Immunity in Lyme Disease 
Lyme disease is a debilitating inflammatory dis- 
ease caused by the spirochete Borrelia burgdorferi 
when transmitted by tick bite to humans and ani- 
mals. Originally considered to be a focal disease in 
small areas of the United States, Lyme disease is now 
realized to be worldwide and has been detected on 
all continents except Antarctica. Protective immu- 
nity was shown by this research group, in collabora- 
tion with the groups of Drs. Fred Kantor and Stephen 
Barthold (both of Yale University) , to be mediated 
by antibody to outer surface protein A and B (OspA 
and OspB), but not the flagellar protein. Vaccina- 
tion with either OspA or OspB fully protected mice 
against Lyme disease, whether transmitted by injec- 
tion or tick bite. This experimental vaccine is 
currently being evaluated for safety prior to human 
clinical trials. For the vaccine to be clinically effec- 
tive, humans must make neutralizing antibodies. Dr. 
Flavell's group has now shown that passive transfer 
of antibodies, from humans that are seropositive for 
OspA and OspB, protects mice from infection with 
Borrelia. This suggests that humans make protective 
antibodies, which enhances the prospects for a hu- 
man vaccine. 
Last year Dr. Flavell's group reported that vacci- 
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