Background: Oral keratinocytes on the mucosal surface are frequently exposed to HIV-1 through contact with infected sexual partners or nursing mothers. To determine the plausibility that oral keratinocytes are primary targets of HIV-1, we tested the hypothesis that HIV-1 infects oral keratinocytes in a restricted manner.

Porphyromonas gingivalis activates protease-activated receptors (PARs) on oral keratinocytes, resulting in downstream signalling for an innate immune response. Activation depends on P. gingivalis gingipains, but could be confounded by lipopolysaccharide signalling through Toll-like receptors. We therefore hypothesized that P. gingivalis cleaves oral keratinocyte PARs in an Arg-(Rgp) or Lys-(Kgp) gingipain-specific manner to upregulate pro-inflammatory cytokines. Immortalized human oral keratinocytes (TERT-2) were incubated with wild-type P. gingivalis (ATCC 33277) or strains from a panel of isogenic gingipain deletion mutants: Kgp-deficient (KDP 129); Rgp-deficient (KDP 133); or Kgp-and Rgp-deficient (KDP 136). After incubation with P. gingivalis, keratinocytes were probed with specific antibodies against the N-terminus of PAR-1 and PAR-2. Using flow cytometry and immunofluorescence, receptor cleavage was marked by loss of specific antibody binding to the respective PARs. TERT-2 cells constitutively expressed high levels of PAR-1 and PAR-2, and lower levels of PAR-3. P. gingivalis ATCC 33277 cleaved PAR-1 and PAR-2 in a dose-dependent manner, while the receptors were unaffected by the protease-negative double mutant (KDP 136) at all m.o.i. tested. The single Kgp-negative mutant preferentially cleaved PAR-1, whereas the Rgp-negative mutant cleaved PAR-2. Wild-type or Kgp-negative mutant cleavage of PAR-1 upregulated expression of IL-1a, IL-1b, IL-6 and TNF-a; the Rgp-negative mutant did not modulate these cytokines. Selective cleavage of PAR-1 on oral epithelial cells by P. gingivalis Rgp therefore upregulates expression of pro-inflammatory cytokines.

Background: Systemic infection with HIV occurs infrequently through the oral route. The frequency of occurrence may be increased by concomitant bacterial infection of the oral tissues, since co-infection and inflammation of some cell types increases HIV-1 replication. A putative periodontal pathogen, Porphyromonas gingivalis selectively up-regulates expression of the HIV-1 coreceptor CCR5 on oral keratinocytes. We, therefore, hypothesized that P. gingivalis modulates the outcome of HIV infection in oral epithelial cells.