REVIEW ARTICLE
Biology of mast cell tryptase
An inflammatory mediator
Jenny Hallgren* and Gunnar Pejler
Department of Molecular Biosciences, The Biomedical Centre, Swedish University of Agricultural Sciences, Uppsala, Sweden
Introduction
Mast cells arise from pluripotent hematopoietic precur-
sors present in the bone marrow [1]. After circulating
in blood, the mast cell precursors home on to various
tissues, where they undergo terminal maturation under
the influence of local growth factors, in particular stem
cell factor. Over the years, it has become widely accep-
ted that mast cells participate in immediate hypersensi-
tivity reactions [1,2], including allergic conditions, but
more recently they have been implicated in a number
of other types of disorder. For example, several lines
of evidence point to a role in autoimmune disorders
such as arthritis, multiple sclerosis and bullous
Keywords
biological function; mast cells; regulation;
tryptase
Correspondence
G. Pejler, Department of Molecular
Biosciences, The Biomedical Centre,
Swedish University of Agricultural Sciences,
Box 575, 751 23 Uppsala, Sweden
Fax: +46 18 550762
Tel: +46 18 4714090
E-mail: Gunnar.Pejler@bmc.uu.se
J. Hallgren, Brigham and Women’s Hospital
and Harvard Medical School, Smith
Research Building, One Jimmy Fund Way,
Boston, MA 02115, USA
Tel: +1 617 525 1290
E-mail: jhallgren@rics.bwh.harvard.edu
*Present address
Brigham and Women’s Hospital and Harvard
Medical School, Smith Research Building,
One Jimmy Fund Way, Boston, MA 02115,
USA
(Received 23 January 2006, accepted
3 March 2006)
doi:10.1111/j.1742-4658.2006.05211.x
In 1960, a trypsin-like activity was found in mast cells [Glenner GG &
Cohen LA (1960) Nature 185, 846–847] and this activity is now commonly
referred to as ‘tryptase’. Over the years, much knowledge about mast cell
tryptase has been gathered, and a recent (18 January 2006) PubMed search
for the keywords ‘tryptase + mast cell*’ retrieved 1661 articles. However,
still very little is known about its true biological function. For example, the
true physiological substrate(s) for mast cell tryptase has not been identified,
and the potential role of tryptase in mast cell-related disease is not under-
stood. Mast cell tryptase has several unique features, with perhaps the most
remarkable being its organization into a tetrameric state with all of the act-
ive sites oriented towards a narrow central pore and its consequent com-
plete resistance towards endogenous macromolecular protease inhibitors.
Much effort has been invested to elucidate these properties of tryptase. In
this review we summarize the current knowledge of mast cell tryptase,
including novel insights into its possible biological functions and mecha-
nisms of regulation.
Abbreviations
BMMC, bone marrow-derived mast cell; CPA, carboxypeptidase A; CS, chondroitin sulfate; DPPI, dipeptidyl peptidase I; IL, interleukin;
MITF, mi transcription factor; mMCP, mouse mast cell protease; NDST-2, N-deacetylase ⁄N-sulfotransferase-2; PAR, protease activated
receptor; PG, proteoglycan; PMMP, pro-matrix metalloprotease; TMT, transmembrane tryptase.
FEBS Journal 273 (2006) 1871–1895 ª2006 The Authors Journal compilation ª2006 FEBS 1871
pemphigoid [3]. Much of this knowledge has been col-
lected through experiments based on mice that are
deficient in mast cells because of a mutation in the
receptor for stem cell factor, the W ⁄W
v
strain [4]. By
comparing the biological responses of wild-type and
mast cell-deficient mice in various animal models of
disease, it has been possible to outline a role for mast
cells in the respective disease.
Although this recent progress has implicated the
mast cell in various settings, it is still unclear how they
contribute to the respective conditions. One clear pos-
sibility is that one or more of the compounds present
in the mast cell secretory granule, which are released
after mast cell stimulation, contribute to a pathological
response. Mast cell degranulation can be triggered
through several mechanisms, the best characterized of
which is cross-linking, by a specific antigen, of IgE
molecules bound to the high-affinity receptor for IgE,
FceRI [5]. However, mast cell degranulation can be
achieved by a variety of other stimuli, e.g. anaphyla-
toxins C5a and C3a, neuropeptides such as substance
P, and engagement of Toll-like receptors [2].
The compounds released on mast cell activation
include a number of preformed substances: histamine,
a number of different cytokines, serglycin proteogly-
cans (PGs) with attached highly sulfated heparin or
chondroitin sulfate (CS) glycosaminoglycan chains,
and various neutral proteases [2]. In addition, mast cell
activation results in de novo synthesis and release of
arachidonic acid-derived mediators, in particular pros-
taglandin D2 and leukotriene C4 [2]. The neutral pro-
teases are further divided into three major classes:
tryptases, chymases and carboxypeptidase A (CPA)
[7,8], of which tryptase is the major type of protease
stored in human mast cell granules [9]. The designation
of tryptase relates to its strong preference for cleaving
substrates at the C-terminal side of Arg and Lys resi-
dues, a substrate specificity that is shared with that of
pancreatic trypsin. Mast cell tryptases have been iden-
tified in various animal species, including human,
mouse, dog, rat, sheep, cow and gerbil, with the phylo-
genetic relationships indicated in Fig. 1. In this review,
the focus will be on current knowledge of murine and
human tryptases.
Human mast cell tryptases
b-Tryptase
b-Tryptase appears to be the main form of tryptase
stored in mast cell granules and is not normally
released into the circulation. However, increased
b-tryptase levels can be found in serum during extreme
inflammatory conditions such as systemic anaphylaxis
[10].To date, three almost identical b-tryptases have
been identified: bI, bII and bIII [11,12](Fig. 2). bI and
bIII differ from bII in that the amino acid at the 142
position in the first two (Fig. 2) is an Asn residue,
whereas bII-tryptase has a Lys residue at this position.
Moreover, Asn142 in bI-tryptase and bIII-tryptase is
part of an N-glycosylation site and therefore these two
enzymes carry an N-linked oligosaccharide at this posi-
tion. The amino acid sequences of bI-tryptase and bII-
tryptase differ only at position 142 (numbering in
Fig. 2). bIII-tryptase is more significantly different in
that positions 60–63 (Fig. 2) are occupied by Arg-Asp-
Arg in contrast with His-Gly-Pro in bI-tryptase and
bII-tryptase (Fig. 2). Evaluation of the substrate spe-
cificity of b-tryptases, by the peptide phage display
technique, has revealed a strong preference for cleaving
substrates with an Arg or Lys residue at the P1 posi-
tion, preference for Lys ⁄Arg in the P3 position, and
some preference for Pro at the P4 position, but with
little specificity at the P2 position [13,14].
Fig. 1. Phylogenetic relationships between mast cell tryptases. Mast cell tryptases from various species were analyzed on the basis of
amino-acid sequences reported to the NCBI database. The phylogenetic tree was created with CLUSTALW using the MEGALIGN program compo-
nent of DNASTAR. The NCBI accession numbers are indicated in parentheses.
Biology of mast cell tryptase - an inflammatory mediator J. Hallgren & G. Pejler
1872 FEBS Journal 273 (2006) 1871–1895 ª2006 The Authors Journal compilation ª2006 FEBS
a-Tryptase
Two very similar a-tryptases have been identified: aI
[15] and aII [16] (Fig. 2). Human a-tryptase was
previously considered unable to be processed into its
mature form [17]. Despite this, recombinant a-tryptase
was shown to be assembled into an active tetramer
(see also below), although the activity was extremely
Fig. 2. Sequence alignment of human and murine mast cell tryptases. The catalytic triad amino-acid residues (His, Asp, Ser) are shown in yel-
low. The proposed cleavage of the signal peptide in human b-tryptase is indicated by a green arrow; the cleavage site in the propeptide is indica-
ted by a red arrow. Conserved His residues that are components of the heparin-binding region, as shown for mMCP-6 [65], are indicated in blue.
N-Glycosylation sites are indicated by green pentagons. The degree of amino-acid sequence conservation among the tryptases are indicated by
different coloring: black (100% conservation), dark grey (80% conservation), light grey (60% conservation), white (< 60% conservation).
J. Hallgren & G. Pejler Biology of mast cell tryptase - an inflammatory mediator
FEBS Journal 273 (2006) 1871–1895 ª2006 The Authors Journal compilation ª2006 FEBS 1873
low compared with b-tryptase [18]. Site-directed muta-
genesis of Asp216 (chymotrypsin numbering; corres-
ponding to Asp255 in Fig. 2) into Gly, which is the
corresponding amino acid in b-tryptase, demonstrated
that the difference in activity was partly attributable to
this amino acid substitution [18]. Further, the crystal
structure of a-tryptase revealed that the substrate-bind-
ing region (Ser214–Gly219; chymotrypsin numbering;
corresponding to Ser253–Gly257 in Fig. 2) is kinked in
the a-tryptase tetramer, which makes substrate binding
and processing unproductive [19]. Using an assay with
monoclonal antibodies that distinguish between a-tryp-
tase and b-tryptase, it was shown that a-tryptase is
present at low levels in the circulation, even in the
absence of mast cell degranulation. This suggests that
a-tryptase is released constitutively, in contrast with
b-tryptase which is stored intracellularly unless the
mast cells have been challenged by a degranulating
agent [10]. Although the reasons for these differences
between a-tryptase and b-tryptase are not clear, it has
been suggested that because of differences in the pro-
peptide of a-tryptase (Gln )3) and b-tryptase (Arg )3)
(Fig. 2), a-tryptase displays defective N-terminal pro-
cessing and as a consequence is continuously secreted
rather than directed to the secretory granules [17].
However, the same authors later showed, using anti-
body-based assays, that precursor forms of both
enzymes are secreted spontaneously [21]. Moreover,
the possibility that the Gln )3 residue in the a-tryptase
propeptide causes defects in processing has been ques-
tioned [18].
c-Tryptase
Two different c-tryptases have been characterized:
cI and cII [21]. These enzymes are expressed in both a
mast cell-like cell line (HMC-1) and airway mast cells
[21]. In contrast with a-tryptases and b-tryptases,
c-tryptases contain an extended hydrophobic C-ter-
minal domain followed by a small cytoplasmic tail.
They are therefore probably transmembrane proteins,
anchored in either the plasma membrane or secretory
granule membranes. Another transmembrane tryptase
(TMT) may be identical to cI-tryptase or at least very
similar (98–99%) [22]. Interestingly, it was demonstra-
ted that TMT migrates to the plasma membrane upon
mast cell degranulation [23]. Hence, TMT ⁄c-tryptase
may be inserted into the secretory granule membrane
in the resting mast cell, with the active site facing the
granular lumen. After mast cell degranulation, the
fusion of the granular membrane with the plasma
membrane results in exposure of TMT ⁄c-tryptase to
the cell exterior. Although the exact consequence of
the membrane insertion of TMT ⁄c-tryptase is not
certain, it is clear that cell surface association of the
protease may serve to localize any biological action
attributable to the TMT ⁄c-tryptase. Interestingly,
when TMT is enzymatically activated it retains its pro-
peptide and forms a disulfide bond linking two TMT
chains together [23]. Although the true biological func-
tion of the TMT ⁄c-tryptase remains to be elucidated,
it is of interest to note that instillation of recombinant
TMT into the trachea of mice results in induction of
airway hyper-responsiveness and interleukin (IL)-13
expression [23].
d-Tryptase
Two nearly identical (differing at only one amino acid
position) tryptases, dI and dII, have been identified [24]
(Fig. 2). d-Tryptase contains a premature stop codon
that results in a shorter mature protein, and it is likely
that this truncation affects the substrate specificity sig-
nificantly [24]. However, despite the premature stop
codon, the catalytic triad is intact, as shown by the abil-
ity of d-tryptase to cleave synthetic peptide substrates
with trypsin-like cleavage specificity (i.e. cleavage on
the C-terminal side of Lys). Further, immunohisto-
chemical analysis has shown that d-tryptase is primarily
expressed by mast cells, in tissues such as colon, lung
and heart as well as in HMC-1 cells [24].
Mouse mast cell tryptases
Mouse mast cell protease (mMCP)-6
mMCP-6 is exclusively expressed in connective tissue-
type mast cells [25]. Phage-display experiments to
define the substrate specificity revealed that mMCP-6
prefers Lys to Arg in the P1 position and has some
preference for Pro in the P4 position, thus closely
resembling the substrate specificity of human b-tryp-
tase [26]. mMCP-6 is a major storage component of
connective tissue-type mast cells, and is not normally
released into the environment. However, on mast cell
degranulation, mMCP-6 can be found in the extracel-
lular matrix that surrounds the degranulated mast cell.
Interestingly, mMCP-6, in contrast with mMCP-7, is
not released into the circulation, but is rather retained
in the vicinity of the degranulated mast cell [27], indi-
cating that it exerts its effect locally.
mMCP-7
mMCP-7 was first discovered in early stage cultures of
bone marrow-derived mast cells (BMMCs) [28]. Later,
Biology of mast cell tryptase - an inflammatory mediator J. Hallgren & G. Pejler
1874 FEBS Journal 273 (2006) 1871–1895 ª2006 The Authors Journal compilation ª2006 FEBS
expression was also found in ear and skin connective
tissues of adult mice [30]. mMCP-7 is highly homolog-
ous with mMCP-6, with 71% amino-acid sequence
identity (Fig. 2). It has been demonstrated to preferen-
tially cleave substrates with Arg in the P1 position and
Ser or Thr in the P2 position [31]. Further, it shows an
unusually high negative net charge at neutral pH ()10).
In contrast with mMCP-6, mMCP-7 can be detected in
plasma as early as 20 min after mast cell degranulation,
probably because of the lack of PG-mediated retention
[27]. This may be explained by the presence of surface-
exposed His residues in mMCP-7 that lose their positive
charge when exposed to neutral pH after exocytosis,
and thereby lose their ability to engage in electrostatic
interactions with anionic mast cell PGs [31]. Interest-
ingly, it has been shown that a commonly used mouse
strain, C57BL ⁄6, lacks mMCP-7 expression because of
a premature stop codon [32]. However, there are no
reports describing any apparent phenotypic effects of
the mMCP-7 deficiency on disease outcome in experi-
mental models for example.
Mouse transmembrane tryptase (mTMT)
mTMT (also referred to as Prss31 [33]) was identified
by mapping the mouse tryptase locus to chromosome
17 [22]. mTMT has a C-terminal hydrophobic exten-
sion similar to cI-tryptase and probably has similar
properties. mTMT expression appears to be strain
dependent; it is expressed in C57BL ⁄6 mice but not
BALB ⁄c or 129 ⁄Sv mice. Moreover, expression seems
to be highest during the early stages of mast cell devel-
opment [22].
mMCP-11
A novel tryptase, denoted mMCP-11 (also referred to
as Pssr34 [33]), has recently been discovered and found
to be expressed in both BMMCs and different mast
cell-like cell lines [34]. Like mTMT, mMCP-11 expres-
sion is highest at early stages of mast cell development.
mMCP-11 has 52% and 54% sequence identity with
mMCP-6 and mMCP-7, respectively, and has trypsin-
like substrate specificity [34].
Gene organization
Human tryptases
The genes for the human mast cell tryptases are clus-
tered together with additional serine protease genes
on chromosome 16 near the end of the short arm
(16p13.3) [16]. Like the b-tryptase (tryptase I) gene
structure, which was the first reported [11], all human
mast cell tryptase genes are composed of six exons sep-
arated by five introns. Organization of the a-tryptase
and b-tryptase genes is very similar but they differ in
that the a-tryptase gene has a 10 or 11-bp deletion in
intron 4 [16]. Interestingly, it has been suggested that
there is one locus for bII and bIII alleles and a separ-
ate locus for bI and aalleles. The discovery that the
aand bI alleles may compete at one locus suggested
that there may be individuals with a complete lack of
a-tryptase [8,16]. Indeed, it was subsequently shown
that a-tryptase deficiency is very common; about 29%
of the human population lack it [35]. An interesting
approach for future research aimed at determining the
biological function of a-tryptase will thus be to investi-
gate if the lack of a-tryptase is associated with a
particular biological response, e.g. increased ⁄reduced
susceptibility to disease. The c-tryptases (I, II) may
also be allelic variants at the same locus [21]. However,
their genetic organization is more closely related to
that of human prostasin [36], a serine protease
expressed in various tissues such as prostate, liver, kid-
ney, lung and pancreas [37]. The genetic organization
of the d-tryptases is similar to that of the a⁄b-tryptas-
es, although the d-tryptase gene contains insertions in
introns 4 and 5, and the 5th exon more closely resem-
bles that of mMCP-7. Because of the latter finding, the
d-tryptases were previously referred to as mMCP-7-like
I and II [16]. The d-tryptases may also be allelic part-
ners [24].
Mouse tryptases
In mouse, the corresponding tryptase locus is located
at chromosome 17A3.3 [34]. This locus contains sev-
eral tryptase genes, although only four of these are
expressed in mast cells: mMCP-6,mMCP-7,mMCP-11
and mTMT.mMCP-6 has six exons ⁄five introns
organized similarly to the a⁄b-tryptase genes, whereas
mMCP-7 has only five exons because of a point muta-
tion which hinders splicing of the region that corres-
ponds to the first intron in mMCP-6 [28]. The mTMT
gene also consists of five exons and, similarly to
human c-tryptases, the last exon codes for a trans-
membrane domain [22]. The newly discovered mMCP-
11 also contains five exons [34].
Regulation of gene transcription
Transcription of tryptase genes is positively regulated
by the mi transcription factor (MITF), a member of
the basic–helix–loop–helix–leucine zipper protein fam-
ily, as first described for mMCP-6 [38]. Other proteins
J. Hallgren & G. Pejler Biology of mast cell tryptase - an inflammatory mediator
FEBS Journal 273 (2006) 1871–1895 ª2006 The Authors Journal compilation ª2006 FEBS 1875