Abstract
Lymphoid cells taken from the grossly enlarged lymph nodes (LN) and spleens of SJL/J mice bearing transplantable reticulum‐cell sarcomas (RCS), showed increased levels of natural killer (NK) cell activity, when compared to LN and spleen cells taken from normal SJL mice. The NK susceptible target cells used for these studies included RL‐1 and YAC‐1. Target cells that were not susceptible to NK lysis by CBA/J effector cells (high NK strain) were also resistant to lysis by RCS tumor cells. Although significant lysis of target cells was detectable at 4 h, optimal NK cytotoxicity was observed in a 16‐h ^51^Cr‐release assay. In terms of lytic unts (LU)/10^7^ effector cells, greater NK activity was observed in RCS effectors than was apparent in normal CBA/J lymphoid cells. Depletion of macrophages from RCS cell preparations had no effect on the observed NK activity. Purified Mu‐IFN‐α stimulated a small increase in the NK activity expressed by SJL lymphoid cells; however RCS‐derived supernatants (containing Mu‐IFN‐γ) did not augment NK activity of SJL effector cells, nor were they cytotoxic for NK target cells. Therefore, tumor‐derived soluble factors appear not to be involved in the observed RCS‐associated NK activity. The data presented in this report further define the parameters of the NK activity manifested by RCS tumor cell preparations. The accompanying article characterizes the nature of the NK effector cells present within RCS tumors of SJL mice. Taken together, these data indicate that the RCS tumors represent either neoplastic expansion of conventional NK cells with somewhat modified properties, or else an entirely new class of cytotoxic effector cells.