Molecular and Cellular Physiology of Acute 
Inflammatory Cytokines 
David D. Chaplin, M.D., Ph.D. — Associate Investigator 
Dr. Chaplin is also Associate Professor of Medicine, Genetics, and Molecular Microbiology at Washington 
University School of Medicine and Assistant Physician at Barnes Hospital, St. Louis. He received his A.B. 
degree in biochemistry from Harvard University and his M.D. and Ph.D. degrees in cellular and 
developmental biology from Washington University. Following a residency in internal medicine at 
Parkland Memorial Hospital, Dallas, he received postdoctoral training in genetics at Harvard Medical 
School with Jonathan Seidman. He then returned to Washington University as a faculty member. 
PROPER regulation of immune defenses re- 
quires the coordinate action of an array of 
different host responses. Many of these responses 
depend for their regulation on the interactions of 
several different types of immune cells. Some of 
these interactions are mediated by physical con- 
tact between the participating cells. Others, how- 
ever, are mediated by soluble proteins that are 
released from one cell to act by binding to spe- 
cific receptors on other cells. These soluble medi- 
ators have been designated cytokines. 
Three cytokines appear particularly important 
for the initiation of acute inflammatory and im- 
mune responses: interleukin-1 (IL-1), tumor ne- 
crosis factor (TNF), and interleukin-6 (IL-6). 
IL-1, TNF, and IL-6 are pleiotropic cytokines. 
They are produced by a wide variety of cells, and 
they act on many tissues, showing a broad array of 
activities. In general, these activities are proin- 
flammatory, favoring first the recruitment of 
white blood cells to sites of inflammation, then 
activation of these cells to express their full ef- 
fector potential. This recruitment and activation 
is absolutely necessary for host defense against 
invading microbes; however, when it occurs inap- 
propriately, it can lead to acute and chronic dam- 
age to self tissues. 
Although a great deal of information has been 
accumulated concerning the activities of these 
cytokines when added to cultured cells or when 
injected into live animals, little is known about 
their true physiological function in normal host 
responses. In the case of IL-1, this uncertainty is 
underscored by studies of the cytokine's struc- 
ture. First, biochemical and molecular genetic 
analysis of IL- 1 has established that there are actu- 
ally two forms of IL- 1 (IL- 1 a and IL- 1 /?) , each the 
product of a discrete gene. The relative produc- 
tion of each form is variable, with some types of 
cells producing primarily IL- 1 a and some primar- 
ily IL-1/3. Both forms are synthesized in the IL-1- 
producing cell as a larger precursor molecule 
(pro-IL-la and pro-IL-1/3), and both are cleaved 
to a smaller mature form that is found outside the 
cell. In vitro, mature IL-1 a and -1|8 show appar- 
ently identical activities. Surprisingly, the two 
cytokines share only 25 percent amino acid se- 
quence identity. Early studies from our labora- 
tory showed that the genes encoding the two IL- 1 
proteins were generated by duplication of an an- 
cestral IL-1 -like gene, with subsequent extensive 
mutation to produce the existing IL-1 a and -1|8 
genes. We interpret the separate lineages of the 
two genes to indicate that there has been selec- 
tive pressure for the independent evolution of 
each gene. Although this pressure probably acts 
at the level of unique functions for each of the 
two IL-1 molecules, in vitro studies have not re- 
vealed any functional diff^erences between the 
two cytokines. This indicates to us that existing 
in vitro models do not adequately represent the 
normal physiology of IL-1 and that new models 
must be explored to disclose the functional dif- 
ferences that must exist in vivo. 
Just as there are fundamental questions con- 
cerning the unique functions of each of the IL- 1 
proteins, there are basic questions concerning 
the mechanism by which these proteins exit the 
cell to perform their extracellular functions. 
Normally proteins that are destined to act outside 
the cell contain a characteristic signal peptide, a 
short amino acid sequence that directs them into 
the cellular secretory pathway. Neither IL- 1 pre- 
cursor protein contains a secretion signal pep- 
tide. Additionally, unlike conventionally se- 
creted proteins, pro-IL-la and -1/5 reside in the 
cellular cytoplasm, not associated with any mem- 
branous cellular compartment. Consequently it 
has been suggested that the IL- 1 proteins are re- 
leased via a novel secretory pathway. 
Recent studies from our laboratory have 
yielded new insights into the mechanism of lL-1 
release and maturation. In these studies we have 
shown that release of IL-1 a and -1/8 from acti- 
vated cells does not involve a unique secretory 
mechanism but occurs as a consequence of injury 
to the IL-1 -producing cell. This conclusion is 
based on the observation that IL-1 release is ac- 
companied by parallel release of other cytoplas- 
mic proteins, such as the enzyme lactate dehydro- 
genase (LDH). Agents that enhance IL-1 release 
either induce increased IL- 1 synthesis or increase 
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