Therefore, one possibility is that the GVT activity of T cells can be negatively regulated by tumors expressing high levels of Ceacam1, but is less important for tumors that express low levels or only on a subset of cells in the first place. However, RENCA in our GVT model systems is found primarily in the liver, and to a lesser extent, the lungs. Since donor allografts with Ceacam1 2/2 T cells showed decreased numbers of donor alloreactive T cells in the liver as compared with wildype in GVHD experiments ( Figure 3A) , interpretation of GVT activity against RENCA with respect to Ceacam1 on T cells must also consider this aspect of its biology.