TNF and periodontitis: As the levels of several inflammatory cytokines, including IL-1β [24] and TNF-α [25] are strongly increased in ligature-induced periodontitis in mice, as well in rats [26], we determined whether PTPα regulates inflammation-mediated destruction of periodontal tissues by placement of silk ligatures around the necks of molar teeth to induce periodontitis [27], [28] using wild type (PTPα+/+) or knock out (PTPα−/−) mice.