Helicobacter spp. represent a proportionately small but significant component of the normal
intestinal microflora of animal hosts. Several of these
intestinal Helicobacter spp. are known to induce
colitis in
mouse models, yet the mechanisms by which these
bacteria induce
intestinal inflammation are poorly understood. To address this question, we performed in vitro co-culture experiments with
mouse and human
epithelial cell lines stimulated with a
selection of
Helicobacter spp., including known
pathogenic species as well as ones for which the
pathogenic potential is less clear. Strikingly, a member of the normal
microflora of rodents,
Helicobacter muridarum, was found to be a particularly strong
inducer of CXC
chemokine (Cxcl1/KC, Cxcl2/MIP-2) responses in a murine
intestinal epithelial cell line. Time-course studies revealed a biphasic pattern of
chemokine responses in these cells, with H. muridarum
lipopolysaccharide (LPS) mediating early (24-48 h) responses and live
bacteria seeming to provoke later (48-72 h) responses. H. muridarum LPS per se was shown to induce CXC
chemokine production in HEK293 cells stably expressing
Toll-like receptor 2 (
TLR2), but not in those expressing
TLR4. In contrast, live H. muridarum
bacteria were able to induce
NF-kappaB reporter activity and CXC
chemokine responses in TLR2-deficient HEK293 and in AGS
epithelial cells. These responses were
attenuated by transient transfection with a
dominant negative construct to
NOD1, and by stable expression of
NOD1 siRNA, respectively. Thus, the data suggest that both
TLR2 and
NOD1 may be involved in innate
immune sensing of H. muridarum by
epithelial cells. This work identifies H. muridarum as a commensal
bacterium with
pathogenic potential and underscores the potential roles of ill-defined members of the
normal flora in the initiation of
inflammation in animal hosts. We suggest that H. muridarum may act as a
confounding factor in
colitis model studies in rodents.
