A successful approach to inhibit proprotein convertases involves genetically engineered antitrypsins, which are derived from a 1antitrypsin (a 1 -AT). a 1 -AT is a serine protease inhibitor (serpin) with a characteristic exposed reactive center loop (RCL), which mediates binding to the active site of its target protease. The exploration for the potential use of modified antitrypsins with an altered inhibitory spectrum has been guided by the discovery of a natural variant of a 1 -AT, known as Pittsburgh (a 1 -AT-PIT), found in a patient who had a severe bleeding disorder caused by mutation of the P1 reactive center residue of antitrypsin from methionine to arginine [37] . This substitution changed its specificity from elastase to thrombin and other coagulation proteases. Due to the introduction of a second mutation from alanine to arginine at P4 of the RCL, the engineered a 1antitrypsin variant Portland (a 1 -AT-PDX) showed high affinity for furin [38] . a 1 -AT-PDX efficiently inhibited the formation of infectious HIV, measles virus, and human cytomegalovirus progeny by blocking furin-dependent processing of glycoproteins gp160, F0 and gB, respectively [38, 39, 40, 41] . Pullikotil and coworkers used this approach for the generation of highly selective a 1 -antitrypsin variants specific for S1P by introducing various S1P recognition motifs into the RCL of a 1 -antitrypsin [42] . The adaptation of a 1 -antitrypsin towards S1P efficiently inhibited the processing of the S1P substrates SREBP-2 (sterol regulatory element binding protein), ATF6 (activating transcription factor 6) as well as CCHFV glycoprotein [42] . However, the effect of these inhibitors on CCHFV infection was not analyzed in that study. To block cleavage of the LASV glycoprotein, we generated here recombinant a 1 -antitrypsin variants mimicking the S1P recognition motifs RRIL, RRVL and RRYL that exhibited the greatest inhibitory potential based on immunoblot quantification. In addition, we used an a 1 -AT construct that contains the LASV GP cleavage motif RRLL in its RCL. Using a doxycycline regulated expression system we demonstrate that S1P-adapted a 1antitrypsin variants efficiently block proteolytic maturation of the glycoprotein precursor GP-C, whereas a furin-specific a 1 -AT had no effect on GP-C processing. Virus replication of both a replication-competent recombinant vesicular stomatitis virus expressing the LASV glycoprotein GP-C (VSVDG/LASVGP) and authentic LASV was significantly inhibited in the presence of S1P-specific a 1 -antitrypsins. The degree of inhibition of viral replication correlated with the ability of the different a 1 -antitrypsin variants to inhibit the processing of LASV GP-C.