The appearance of VIRIP in plasma samples from HIV-1infected subjects raised the question of how proteolytic processing of AAT may be mediated under these conditions. Previous studies indicated that MMPs (collagenases and elastases) interact with and cleave AAT [20] . Inspection of the regions of AAT flanking the VIRIP sequence predicted cleavage sites for MMP-1, -6, -7, -8, -9, -12, and MMP-26 at the N-terminal, and MMP-7 at the Cterminal end of VIRIP (Fig. 5A) . In vitro digestion of purified AAT with recombinant MMP-7 revealed a degradation product at 5 kDa detectable by immunoblotting consistent with the Cterminal AAT fragment 377-418 containing VIRIP (Fig. 5B) . Subsequent analysis was carried out using LC-MS/MS, which identified peptides containing both cleavage sites required for the formation of VIRIP at Phe 376 -Leu 377 and Phe 396 -Leu 397 (Fig. 5C) . A third cleavage within the VIRIP sequence at Pro 381 -Met 382 was also observed. Importantly, formation of VIRIP itself was detected at the 2 h time point, confirming MMP-7 as a candidate protease involved in generation of the peptide in vivo. Antiviral activity of factors elevated in the plasma during AHI In order to test for potential interference with the infection process we evaluated the ability of acute phase proteins AAT, A-SAA and C-reactive protein (CRP) and the C-terminal AAT fragments 397-418 and 377-396 (VIRIP) to inhibit HIV-1 replication using in vitro peripheral blood mononuclear cell (PBMC) and dendritic cell (DC) infection models. PHA-activated PBMCs or monocyte-derived dendritic cells (MDDCs) were incubated with the test analytes both prior to and during infection with either an R5-or an X4-tropic virus and subsequent HIV-1 replication was monitored by the analysis of supernatant p24 levels or reverse transcriptase activity. VIRIP, derived from near the Cterminus of AAT, markedly inhibited the replication of both the R5 and the X4 virus, consistent with previous findings [10] (Fig. 6A) . In contrast, no effect was observed with either fulllength AAT or the C-terminal 22mer 398-418 (Fig. 6A ). In addition, CRP did not inhibit the replication of either the R5 or the X4 virus in PBMCs (Fig. 6C) . A-SAA did not exhibit any inhibitory activity in the PBMC infection system, but did inhibit the replication of the R5 virus in MDDCs as early as 12-24 h after infection (Fig. S3) and to a greater extent after 7 d (Fig. 6B) . Importantly, inhibition of MDDC infection was still greater than 50% at a 1mgml 21 A-SAA concentration, which is well in the range of the levels detected in infected individuals (Fig. 3A) . A-SAA was recently reported to inhibit MDDC infection by an X4/ R5 dual tropic virus via down-regulation of CCR5 expression [19] . We conclude that components of the acute phase response indeed have the capacity to interfere with HIV-1 infection, thereby potentially helping to control viral dissemination in the eclipse phase.