Mast cells release potent mediators upon degranulation, including serine proteinases.
These proteinases play a pivotal role in the pathogenesis of human asthma. Due to
the similarities between human asthma and equine heaves, a similar role for the mast
cell in equine heaves is proposed.
Antibodies directed against equine tryptase and equine mast cell proteinase-1
(eq.MCP-1) were purified, validated and used to develop ELISAs for determination
of proteinase concentrations in bronchoalveolar lavage fluid (BALF) from controls,
clinical heaves horses, heaves horses in remission and horses with other pulmonary
diseases. Clinical heaves horses had significantly increased BALF tryptase
concentrations compared to controls or heaves horses in remission, whereas BALF
tryptase concentrations of controls and heaves horses in remission did not
significantly differ. Horses with other pulmonary diseases also had significantly
elevated BALF tryptase concentrations compared to controls. Very low eq.MCP-1
concentrations were detected in all samples. Likewise, eq.MCP-1 immunoreactive
cells were scarcely observed in equine lung tissue or BALF cytospins suggesting that
eq.MCP-1 may be unimportant in the healthy and heaves affected equine lung.
Cloning and sequencing of these proteinases revealed an alanine 216 substitution in
equine tryptase, which confers increased arginine substrate specificity and may
restrict fibrinogenolysis in vivo. The deduced eq.MCP-1 Aa sequence differed from
that isolated from equine mastocytoma tissue and it appears that a similar, but novel,
chymase was sequenced. Probing of tryptase mRNA transcript regulation in control
and heaves susceptible horses revealed no significant change in airway luminal cell
pellet tryptase expression following hay/straw challenge of control or heaves horses.
However, bioiicliiolar tissue from heaves horses in early resolution phase had
significantly down-regulated tryptase transcripts compared to controls. Furthermore,
immunohistochemistry revealed significant intra-epithelial recruitment of tryptase
positive mast cells in heaves horses compared to controls, suggesting involvement of
tissue mast cells in response to challenge.
In vitro hay dust suspension (HDS) challenge induced significant airway luminal
mast cell degranulation in heaves susceptible horses, however a similar dose
response trend was also evident in control horses. The increased number of intra¬
epithelial mast cells in heaves horses may explain the divergent mast cell response to
in vivo and in vitro challenges. HDS-induced mast cell degranulation in both control
and heaves horses may suggest non-IgE mediated degranulation. Alternatively, both
control and heaves horses may have been sensitised to HDS allergens and phenotypic
diversity may ultimately determine response to challenge. Collectively, these results
provide evidence for mast cell involvement in the pathogenesis of equine heaves and
warrant further studies into the potential roles of mast cell mediators.