Natural killer cell activity and serum autoantibodies in women with postpartum thyroiditis

Postpartum thyroiditis (PPT) is a form of painless lymphocytic thyroiditis which is thought to be caused by an exacerbation of underlying subclinical autoimmune thyroid disease during a postpartum period of immune rebound. The purposes of this study were to analyze natural killer cell (NK) functional activity and antibody-dependent cell-mediated cytotoxicity (ADCC) of peripheral blood mononuclear cells in patients with PPT compared to those in normal nonpostpartum and postpartum women. NK activity and ADCC were determined by specific lysis of 51Cr-labeled K562 human erythroleukemia tumor cells at varying effector (mononuclear cell) to target cell ratios. Nineteen PPT patients between 4-9 months [mean, 6.5 +/- 0.4 (+/- SE)] postpartum were compared to 20 nonpostpartum women, 31 healthy women between 4-9 months (6.7 +/- 0.3) postpartum, and 14 women 2 days after delivery. There were significant differences in the NK functional activities of the 4 groups (F = 7.95; P = 0.0002). The mean NK activities, as measured by percent specific lysis at an effector to target cell ratio of 10:1, were 31.1 +/- 3.3%, 20.9 +/- 2.3%, 22.5 +/- 2.8%, and 11.5 +/- 2.0% in the nonpostpartum, PPT, postpartum, and 2-day postpartum groups, respectively. Specific lysis by ADCC was not significantly different from lysis by NK activity in any group. The PPT, postpartum, and 2-day postpartum groups had significantly lower NK activity compared to that in nonpostpartum women (P less than 0.05). Both the PPT and postpartum women had higher NK activities than the 2-day postpartum women (P less than 0.05). However, there were no significant differences in the NK activities of the PPT patients compared to those of the healthy postpartum women. Patients with PPT were also found to have associated autoimmune dysfunction. The PPT group had significantly higher serum antinuclear antibody titers; 8 of 24 patients (33%) had titers of 1:160 or greater compared to only 2 of 29 healthy postpartum women (7.4%; P less than 0.05). Seven of the PPT patients had serum immune complexes, and 3 had TSH receptor antibodies. We conclude that functional NK activity and ADCC in peripheral lymphocytes of PPT patients are not different from those in healthy postpartum women; however, the postpartum women had significantly decreased activity compared to that in nonpostpartum women. These data emphasize the importance of studying healthy postpartum women in investigations of PPT, since the immunological changes in pregnancy and the postpartum period remain largely undefined.