creases the number of eosinophils. In contrast, THl 
cells secrete interferon-7 and lymphotoxin, which 
activate the killing functions of macrophages and 
granulocytes. 
A few years ago a cDNA library made from induced 
TH2 cells was used to isolate a clone, P600, that was 
expressed in activated but not resting TH2 cells. The 
P6OO sequence had no homology to other se- 
quences in nucleic acid databases, and the open 
reading frame potentially encoded a 131-amino 
acid protein that contained a short hydrophobic se- 
quence at the amino terminal, suggesting a secreted 
protein. P6OO mRNA was synthesized after activa- 
tion ofTH2 but not THl cells. All of this information 
is consistent with the hypothesis that the P600 pro- 
tein is a new TH 2 -specific cytokine. Because all of 
the other TH2-specific cytokines play a major role in 
the unique functions of TH2 cells, Dr. Mosmann's 
laboratory has been investigating the functions of 
P600 to determine how this potential new cytokine 
fits into the overall picture of TH1/TH2 immuno- 
regulation. 
A second P600 cDNA clone was isolated from an- 
other TH2 cDNA library, and the two clones were 
expressed by transfection into COS cells. The super- 
natants of such transfections contained a new 14- 
kDa protein, consistent with the length of the P600 
open reading frame, assuming that the hydrophobic 
leader sequence was cleaved and a small amount of 
carbohydrate added. These transfection superna- 
tants were tested in a variety of bioassays; in contrast 
to many T cell-derived cytokines, P6OO appears to 
have a limited range of activities. The main activity 
discovered so far is the stimulation of a precursor 
cell in bone marrow to proliferate and differentiate 
into cells with macrophage-like properties. 
The morphology of these cells is similar to the 
adherent cells that result from treatment of bone 
marrow with granulocyte macrophage colony- 
stimulating factor (GM-CSF) but distinct from the 
morphology of macrophages derived by growth of 
bone marrow cells in macrophage CSF (M-CSF) . The 
precursor frequency of the P6 00 -responsive cell is 
much lower than that of either the M-CSF- or GM- 
CSF-responsive precursors. Many of the P600- 
derived cells bear the MAC-1 and F4/80 surface an- 
tigens that are normally present on macrophages. 
In functional assays the cells that result from 
growth in P6OO are relatively well differentiated, as 
measured by their ability to cleave nitroblue tetrazo- 
lium much more effectively than GM-CSF- or M- 
CSF-derived cells. The P600-derived adherent cells 
also efficiently present alloantigens and soluble 
protein antigens to T cell clones, supporting their 
designation as a type of macrophage. However, the 
P600 cells show little or no phagocytosis of anti- 
body-coated sheep erythrocytes, in contrast to the 
cells resulting from M-CSF treatment. 
In general, THl cells produce cytokines that ac- 
tivate macrophages, whereas TH2 cells produce 
cytokines, such as IL-4 and IL-10, that inhibit 
macrophage cytotoxic functions. Thus it is not im- 
mediately apparent how P600, which appears to 
stimulate the growth of a particular type of macro- 
phage, fits into the set of functions typical of TH2 
cells. It is possible that P6OO increases the number 
of macrophages without causing activation or that 
the P600-derived cells are a unique type of macro- 
phage with an undiscovered role in the immune sys- 
tem or hemopoiesis. Studies are continuing on the 
nature of the P600-responsive cells and the effects 
of P6OO treatment in vivo. 
Dr. Mosmann is also Professor and Chair of the 
Department of Immunology at the University of 
Alberta, Edmonton. 
CELLULAR AND MOLECULAR MECHANISMS OF VIRULENCE EXPRESSION 
IN ENTAMOEBA HISTOLYTICA 
Esther Orozco, Ph.D., International Research Scholar 
The long-range objective of Dr. Orozco's group is 
to understand the regulation of the molecular mech- 
anisms involved in the expression of the virulence 
of Entamoeba histolytica, the protozoan parasite 
responsible for human amebiasis. They have de- 
scribed several molecules involved in the cytolytic 
activity of this intestinal parasite and are currently 
cloning and studying these molecules. Some are dif- 
ferentially expressed in pathogenic and nonpatho- 
genic trophozoites. Dr. Orozco and her colleagues 
are focusing on the molecular basis of these differ- 
ences. They also wish to determine the phyloge- 
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