Kinins, the vasoactive
peptides proteolytically liberated from
kininogens, were recently recognized as signals alerting the innate immune system. Here we demonstrate that Leishmania donovani and Leishmania chagasi, two etiological agents of
visceral leishmaniasis (VL), activate the
kinin system. Intravital microscopy in the hamster cheek pouch showed that topically applied promastigotes induced macromolecular leakage (
FITC-dextran) through postcapillary venules. Peaking at 15 min, the parasite-induced leakage was drastically enhanced by
captopril (Cap), an inhibitor of
angiotensin-converting enzyme (ACE), a
kinin-degrading
metallopeptidase. The enhanced microvascular responses were cancelled by
HOE-140, an antagonist of the B2
bradykinin receptor (B2R), or by pre-treatment of promastigotes with the irreversible
cysteine proteinase inhibitor N-methylpiperazine-
urea-Phe-homoPhe-vinylsulfone-
benzene (N-Pip-hF-VSPh). In agreement with the above-mentioned data, the promastigotes vigorously induced
edema in the paw of Cap-treated J129 mice, but not Cap-B2R-/- mice. Analysis of parasite-induced breakdown of
high molecular weight kininogens (HK), combined with active site-affinity-labeling with
biotin-N-Pip-hF-VSPh, identified 35-40 kDa
proteins as
kinin-releasing
cysteine peptidases. We then checked if macrophage infectivity was influenced by interplay between these
kinin-releasing parasite
proteases,
kininogens, and
kinin-degrading
peptidases (i.e. ACE). Our studies revealed that full-fledged B2R engagement resulted in vigorous increase of L. chagasi uptake by resident macrophages. Evidence that inflammatory macrophages treated with
HOE-140 became highly susceptible to amastigote outgrowth, assessed 72 h after initial macrophage interaction, further suggests that the
kinin/B2R activation pathway may critically modulate
inflammation and innate immunity in
visceral leishmaniasis.