Blood-feeding insects inject potent salivary components including complement inhibitors into their host's skin to acquire a blood meal. Sand fly saliva was shown to inhibit the classical pathway of complement; however, the molecular identity of the inhibitor remains unknown. Here, we identified SALO as the classical pathway complement inhibitor. SALO, an 11 kDa protein, has no homology to proteins of any other organism apart from New World sand flies. rSALO anti-complement activity has the same chromatographic properties as the Lu. longipalpis salivary gland homogenate (SGH)counterparts and anti-rSALO antibodies blocked the classical pathway complement activity of rSALO and SGH. Both rSALO and SGH inhibited C4b deposition and cleavage of C4. rSALO, however, did not inhibit the protease activity of C1s nor the enzymatic activity of factor Xa, uPA, thrombin, kallikrein, trypsin and plasmin. Importantly, rSALO did not inhibit the alternative or the lectin pathway of complement. In conclusion our data shows that SALO is a specific classical pathway complement inhibitor present in the saliva of Lu. longipalpis. Importantly, due to its small size and specificity, SALO may offer a therapeutic alternative for complement classical pathway-mediated pathogenic effects in human diseases.

Biology Center of the Czech Academy of Sciences Budweis CZ 370 05 Czech Republic

Department of Medical Microbiology and Immunology University of Toledo College of Medicine and Life Sciences Belo Horizonte MG Brasil

Laboratório de Fisiologia de Insetos Hematófagos Departamento de Parasitologia Universidade Federal de Minas Gerais Belo Horizonte MG Brasil

The University of Texas Health Science Center at Tyler Tyler TX

Vaccine Formulation Laboratory Department of Biochemistry University of Lausanne Switzerland

Vector Biology Section LMVR National Institute of Allergy and Infectious Diseases NIH Rockville MD

Vector Molecular Biology Section LMVR National Institute of Allergy and Infectious Diseases NIH Rockville MD

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