To initiate an efficient bacterial infection, microbes require the binding and in some cases invasion into host cells. To date, BslA, an S-layer protein has been identified as a Bacillus anthracis adhesin. Here, we investigated the role of immune inhibitor A metalloprotease (InhA) of B. anthracis in adhesion and invasion in endothelial cells. We determined that InhA can serve as a negative regulator of adhesion and invasion in human brain endothelial cells (hBMECs) using wildtype Sterne and inhA-deficient (ΔinhA) bacilli. The complementation of inhA into mutants induced a similar level of binding as wildtype. BslA levels in S-layer extracts of ΔinhA strain were significantly higher than wildtype and complemented strains. An inverse correlation between InhA and BslA expression was observed when bacilli were cultured in air or bicarbonate/CO₂ conditions. Furthermore, BslA was sensitive to purified InhA degradation in a concentration- and time-dependent manner and transcriptional changes in the BslA gene were not observed. Invaded wildtype bacilli were resistant to intracellular killing compared to ΔinhA strain. Finally, we found InhA serves an important factor to anthrax virulence in late stage disease in mice. Taken together, these results suggest InhA regulates B. anthracis adhesion by modifying cell surface properties through direct proteolysis of adhesin protein, BslA and bacteria expressing a hyperadhesive phenotype exhibit decreased virulence in mice.