Differences in innate immune responses upon stimulation with gram-positive and gram-negative bacteria

Host recognition pathways for gram-negative and gram-positive bacteria comprise pattern recognition receptors among which Toll-like receptors (TLRs) play a pivotal role. TLRs share common signaling pathways yet exhibit specificity as well. Periodontal disease is initiated and maintained in the first line by gram-negative but also gram-positive bacterial infection of the gingival sulcus. To date only limited information is available on whether gram-positive and gram-negative bacteria induce different host responses (strength or quality).

To elucidate these differential effects we focused on proinflammatory cytokine releases by assessing ex vivo stimulation of whole blood with heat-killed gram-negative and gram-positive bacteria and thereof derived microbial products associated with distinct TLRs. Tumor necrosis factor-alpha and interleukin-8 release were measured in the supernatants by enzyme-linked immunosorbent assay. In addition, innate immune responses of peritoneal macrophages from mice lacking TLR2 and TLR4 were tested.

We observed that gram-negative and gram-positive species induced distinct patterns of cytokine production. Gram-negative species produced higher amounts of tumor necrosis factor-alpha while gram-positive species released higher amounts of the chemokine interleukin-8. Data from TLR knockout mice and TLR-transfected HEK cells revealed a somehow specific role of TLR4 and TLR2 for the recognition of gram-negative and gram-positive bacteria, respectively, an observation that goes along with the dominant recognition of the respective pathogen associated molecular patterns lipopolysaccharide and lipoteichoic acid.

The results show that gram-negative and gram-positive bacterial species induce different patterns of immunoregulatory activity, which might be the result of activation of different TLRs.