clones using yeast artificial chromosome (YAC) vec- 
tors. Studies have been initiated to apply these YAC 
clones to identify unrecognized genes within the 
HLA complex. Initial studies have been performed 
in a directed fashion, in which CpG islands and re- 
striction fragments that show strong cross-species 
nucleotide sequence conservation have been used 
as possible markers of genes. 
This approach has led to the identification of a 
novel gene located telomeric of HLA-C that is ex- 
pressed exclusively in keratinocytes. The gene di- 
rects the transcription of a 2.6-kb mRNA that is ex- 
pressed at high levels in normal human skin and at 
lower levels in transformed keratinocyte cell lines, 
but is absent from bone marrow-derived cells, fibro- 
blasts, liver, kidney, muscle, lung, and brain. It pre- 
dicts a protein product of 49 kDa with 28% serine 
residues. It shows no clear amino acid sequence ho- 
mology to previously analyzed proteins. Its possible 
contribution to the HLA-C-linked dermatologic dis- 
order psoriasis vulgaris is under investigation. 
Complementary studies are under way using more 
systematic methods to assess gene content. In collab- 
oration with Drs. Sherman Weissman and David 
Schlessinger, Dr. Chaplin's laboratory is using puri- 
fied DNA from individual YACs to select clones from 
groups of cDNA libraries. The selected cDNAs are 
recovered in clonable form using PCR (polymerase 
chain reaction). In an initial effort using a YAC that 
spans the 400-kb interval from HLA-DRA to the 
2 1 -hydroxylase locus, five new genes have been 
identified, two of which are expressed in B lympho- 
cytes and three of which are not. This selection 
methodology is powerful and rapid and is only lim- 
ited by the quality of cDNA libraries available. Fu- 
ture extensions of this effort should permit the defi- 
nition of the entire gene content of the HLA 
complex. (Portions of these studies using cDNA se- 
lection have been supported by a grant from the Na- 
tional Institutes of Health.) 
Characterization of Mouse IL-1 
For almost two decades it has been recognized 
that IL-1 is an inducible cellular protein that can 
potently modulate the function of most of the effec- 
tor cells of the immune system as well as a broad 
spectrum of other cell lineages. In vitro studies 
have indicated that IL-1 has broad pro-inflammatory 
activities and can modulate antigen-specific im- 
mune responses. Molecular genetic and cell biologi- 
cal analyses have demonstrated that there are two 
isoforms of IL-1, designated IL-1 a and 1L-1;S, en- 
coded by separate, closely linked genes. Both pro- 
teins are synthesized as ~31-kDa intracellular pro- 
molecules and are found in culture supernatants as 
processed 17-kDa mature proteins. For IL-la, both 
the 31 -kDa pro-form and the 17-kDa mature form 
are bioactive. In contrast, pro-lL-l(8 must be 
processed to the 17-kDa mature form to acquire 
receptor-binding activity. 
Previous studies in Dr. Chaplin's laboratory dem- 
onstrated that in conventional in vitro models of 
immune activation, processing and release of IL-1 
from the IL- 1 -producing cells was inefficient, at lev- 
els consistent with release occurring due to cellular 
injury. Recent studies have shown that in the case of 
IL-1|8, the nature of the cellular injury dramatically 
influences the outcome of IL-1 release. Injuries 
characterized by cellular necrosis lead to release of 
unprocessed, inactive pro-IL-l|8, whereas cellular 
injuries leading to programmed cell death cause 
rapid, efficient conversion of pro-IL-l/? to its active 
form. These data suggest that the primary function 
of IL-1 may be to signal cellular injury and to acti- 
vate a systemic response. 
Because the conversion of IL- 1 /3 from its inactive 
pro-form to the active mature form is a critical regu- 
latory step in activation of the lL-1 response. Dr. 
Chaplin's laboratory has initiated studies to define 
the mechanisms by which this conversion is con- 
trolled. Studies using lysates of lL-1 -producing 
cells show that pro-IL-1/3 is activated by a novel 
protease (designated the IL-1;S convertase). In col- 
laboration with Doug Cerretti, Dr. Chaplin's labora- 
tory has isolated cDNA clones encoding the mu- 
rine convertase and has determined its primary 
structure. 
This enzyme is a novel cysteine protease that also 
has some structural features characteristic of serine 
proteases. At the mRNA level, it is constitutively ex- 
pressed in a broad spectrum of cell types; however, 
its expression alone does not confer IL- 1 |8-convert- 
ing activity. Macrophages expressing the convertase 
and pro-IL- 1 /3 together show no IL- 1 18 activation un- 
less cellular apoptosis is induced. Preliminary data 
suggest that the primary translation product of the 
convertase is a proteolytically inactive 4 5 -kDa 
proenzyme. The purified active convertase is a 22- 
kDa/lO-kDa heterodimer. 
Activation of cells to undergo apoptosis increases 
the convertase activity recovered in cell lysates. 
This suggests that activation of the apoptotic pro- 
gram constitutes a primary signal for conversion of 
the convertase from a latent cellular pro-form to its 
catalytically active form. Future studies will docu- 
ment the molecular form of the convertase in viable 
resting and activated cells and in injured cells un- 
dergoing necrosis or apoptosis. (Initial studies of 
IMMUNOLOGY 317 
