Despite the important role that TLR4 plays in mediating the response to LPS in mammals, other genes are involved in this complex pathophysiologic response. Although the mutations in TLR4 are associated with LPS hyporesponsiveness in humans21 and mice,26–28 not all of our subjects who were hyporesponsive to LPS had the mutations in TLR4, and not everyone with the TLR4 mutations was hyporesponsive to inhaled LPS. Moreover, our preliminary data in mice indicate that LPS responsiveness is, in part, determined by mutations in genes other than TLR4. C3H/HeJ was the first mouse strain shown to be LPS hyporesponsive.29 C3H/HeJ mice have a mutated TLR4 protein, which shows no activity in vitro and C3H/HeJ-derived B cells and macrophages show an almost identical LPS response compared to TLR4 -/- derived B cells. Yet, C3H/HeJ mice are able to respond to LPS.30 First, in vivo studies demonstrate that C3H/HeJ mice are hyporesponsive, not unresponsive to LPS, and will initiate LPS-dependent transcription at high doses of LPS.31 Second, the defects in response to LPS can be overcome by treatment with interferon-γ and other agents.32,33 Third, certain LPS preparations, such as isolated from Porphyromonas gingivalis, can activate C3H/HeJ mice,34 suggesting that different receptors may exist for different forms of LPS. In fact, while C3H/HeJ mice were unresponsive to smooth LPS, they showed a normal response to rough LPS.35 Fourth, it was hypothesized that two unique receptors would exist for each isoform of LPS. A 38-kd protein was identified as being present on both normal and C3H/HeJ lymphoid cells, and binding of rough LPS to this protein could not be inhibited by purified lipid A, as had been the case for an 80-kd receptor molecule.36,37 In addition, major histocompatibility complex class II genes have been shown to modulate the response of macrophages to LPS.38 Human and murine cells with reduced expression levels of class II major histocompatibility complex molecules showed a decreased secretion of proinflammatory cytokines following LPS stimulation. Fifth, recent experiments in CD14-/- mice have shown that while CD14 is essential for the cellular response to LPS, in CD14-/- mice, Escherichia coli can stimulate TNF-α secretion at similar levels as wild-type mice. Part of this CD14-independent response appears to be mediated by CD11b.39,40 These results suggest that exposure to bacteria will cause activation of cellular pathways in addition to the ones activated by LPS, and that in the absence of the primary receptor molecule, in this case CD14, other proteins can substitute and initiate alternative signaling pathways. Finally, the strongest indication that genes independent of TLR4 are involved in LPS response comes from a comparison of macrophages and splenocytes derived from TLR4-/- and MyD88-/- mice. The MyD88 gene encodes a protein acting downstream of TLR4 and is thought to complex with TLR4 on LPS stimulation. While MyD88-/- mice-derived cells were totally unresponsive to LPS, TLR4-/- derived macrophages and splenocytes were able to respond to P gingivalis LPS and Gram-positive cell wall components,41 similar to observations in C3H/HeJ mice. These findings indicate that in the absence of functional TLR4, cells tend to be hyporesponsive rather than unresponsive, suggesting the presence of genes, other than TLR4, mediating the response to LPS.