isoform, where all three variable exons have been 
removed. Thl and Th2 cells also differ predictably 
in their expression of CD45 isoforms, suggesting a 
correlation betw^een isoform expression and func- 
tion/differentiation of a CD4 T cell. 
Studies carried out previously in Dr. Bottomly's 
laboratory had shown that CD45 may influence the 
interaction of the T cell receptor with its CD4 co- 
receptor, and thus control signal transduction 
through the receptor. On memory CD4 T cells, 
CD4, CD45, and the T cell receptor appear to be 
associated, behaving as a single unit on the T cell 
surface; whereas on naive CD4 T cells, the three 
molecules migrate independently. These studies 
suggested that CD45 mediates the interplay be- 
tween the T cell receptor and the CD4 co-receptor, 
and might influence signal transduction, perhaps 
mediating the differences in signal transduction be- 
tween memory and naive T cells and between Th 1 
and Th2 cells. 
To explore these issues. Dr. Bottomly has ex- 
pressed a well-characterized T cell receptor, to- 
gether with CD4 or CD45 or both, in a CD45- 
negative T cell thymoma line. Thus, using CD45 
cDNAs to restore CD45 expression, cells bearing no 
CD45 or CD45 of one or more defined isoforms can 
be produced. Preliminary experiments indicate that 
the T cell receptor expressed alone is able to trans- 
duce signals in response to ligation with immobi- 
lized anti-T cell receptor monoclonal antibodies. 
CD45 transfection into cells expressing only the T 
cell receptor appears to have no effect on the re- 
sponse to T cell receptor ligation. However, when 
CD4 is expressed in cells having the T cell receptor 
without CD45, ligation of the T cell receptor no 
longer transduces signals measurable by cytokine 
release. This is surprising because this T cell recep- 
tor has been shown to be more effective at signaling 
in cells that also express CD4. 
These prior studies were carried out in cells with 
mixtures of CD45 isoforms expressed. This suggests 
that CD45, which has been shown to play a critical 
role in signal transduction in T cells, acts on CD4, 
and biochemical data from other laboratories sup- 
port this hypothesis. Studies on cells transfected 
with the T cell receptor, with CD4 , and with individ- 
ual CD45 isoforms are in progress. These should re- 
veal whether CD45 is required for signaling in these 
cells, and also whether individual CD45 isoforms 
have distinctive effects on signal transduction. More- 
over, CD45 mutants lacking tyrosine phosphatase 
activity or ectodomains will allow an examination 
of the role of different parts of this molecule in such 
transfectants. 
Differentiation of T Cells in Murine 
and Human Thymus 
Before T cells can be accessed by antigen in lym- 
phoid tissues leading to adaptive immunity, they 
undergo a process of differentiation and selection 
within the thymus. The diflferentiative pathway ap- 
pears to be preprogrammed within the cell, while 
selection acts on the expressed receptor. Dr. Bot- 
tomly's laboratory has examined the expression of 
receptors and of cytokines in human and murine 
thymus, using ontogenetic development to attempt 
to map out the sequential expression of receptor 
and cytokine genes. 
The T cell developmental lineage splits very early 
into T cells expressing yd receptors and those ex- 
pressing the common al3 receptor found on most 
peripheral T cells. However, early in ontogeny, yd T 
cells dominate the intrathymic T cell population in 
mice, expressing a limited number of receptors. Us- 
ing in situ hybridization to examine expression of 
75 receptor genes. Dr. Bottomly's studies showed 
early fetal expression of yb receptors. This pattern 
of early 7^ gene expression was not observed in hu- 
man thymus, but more detailed analysis of these 
genes by cDNA sequencing demonstrated that the 
receptors found early in human ontogeny showed a 
sequence pattern similar to that found in the mouse. 
Thus there appears to be a primitive T cell receptor 
with very simple sequences encoded by 75 genes 
that arises first in ontogeny, and perhaps preceded 
the a/? receptor in phylogeny as well. Interestingly, 
yd T cells, whose function is not yet clear, were 
shown to play a significant role in pulmonary infec- 
tion with influenza virus, supporting a role of these 
cells in epithelial defense. 
Analysis of cytokine gene expression in thymus 
using in situ hybridization shows that IL-2 and IL-4 
genes are expressed very early in ontogeny in the 
thymus, but this burst of cytokine production is rap- 
idly extinguished during development. The expres- 
sion of these cytokines and of the receptors for them 
is coordinated. However, the precise role of these 
cytokines in T cell development is not yet under- 
stood, especially since it has been reported that 
mice lacking IL-2 or IL-4 have normal T cell develop- 
ment. These studies do show that early in T cell de- 
velopment, IL-2 and IL-4 can be expressed coordi- 
nately in T cells, thus suggesting that the separation 
in production of the cytokines seen in mature effec- 
tor cells using the same technique is a diflferentia- 
tive event in T cells. 
Finally, the expression of CD45 isoforms in T cell 
development might be expected to play a role in 
selective events, since CD45 interacts with CD4, 
IMMUNOLOGY 315 
