Preeclampsia (PE) is associated with altered immune activation during pregnancy. We have previously shown that adoptive transfer of CD4⁺ T cells from the reduced uterine perfusion pressure (RUPP) rat model of PE increases blood pressure, oxidative stress (ROS), and inflammation in normal pregnant recipient rats. The objective of this study was to determine if blockade of communication via the CD40-CD40 ligand interaction between placental ischemia-induced CD4⁺ T cells with endogenous normal pregnant (NP) cells would improve pathophysiology that was previously observed in NP recipient rats of RUPP CD4⁺ T cells. Splenic CD4⁺ T lymphocytes were magnetically separated, incubated with 2.5 µg/mL anti-CD40 ligand (αCD40L) overnight and transferred into NP rats on day 12 of gestation (NP+RUPP CD4⁺T cells+CD40L). On day 19 of gestation, blood pressure (MAP), blood, and tissues were collected. MAP was 99±2 in NP (n=13), 116±4 in NP+RUPP CD4⁺ T cells (n=7; p<0.01); MAP only increased to 104 ±2 in NP+RUPP CD4+Tcells+CD40L (n=24) (p<0.05 vs NP+RUPP CD4⁺T cells). Mechanisms of hypertension in response to RUPP CD4⁺ T cells include ET-1, ROS, and AT1-AA were analyzed. Inhibition of CD40 ligand binding reduced placental ET-1 to 2.3 fold above NP rats, and normalized placental ROS from 318.6 ±89 in NP+RUPP CD4⁺T cells (p<0.05) to 118.7 ±24 in NP+RUPP CD4⁺T cells+CD40L (p<0.05). AT1-AA was also normalized with inhibition of CD40 ligand. This data suggests that placental ischemia-induced T cell communication via the CD40 ligand is one important mechanism leading to much of the pathophysiology of preeclampsia.