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Background Monoclonal antibodies (mAbs) can trigger malignant cell killing through NK cell-mediated antibody-dependent cellular cytotoxicity (ADCC). T cells can be retargeted using CD3-based bispecific antibodies (bsAb) to induce malignant cell death. Both mAb that mediate NK cell ADCC and bsAb that retarget T cells are of enormous clinical value. Our prior studies suggest potential synergy between these two antibody-based therapeutic approaches. 1–3 To assess this synergy further, we developed a novel model that allows for analysis of direct versus indirect effects of mAb and bsAb-mediated NK cell and T cell activation and cytotoxicity.Methods Wild type Raji lymphoma cells express both CD20 and CD38. CRISPR-Cas9 was used to knock out Raji cell CD20 or CD38. The resulting Raji cells expressed either CD20 or CD38 but not both. Target cells consisted of CD20+ Raji alone, CD38+ Raji alone or a mixture of CD20+ Raji and CD38+ Raji. Effector cells consisted of 1) PBMCs depleted of T cells with various concentrations of T cells added back or 2) PBMCs depleted of NKs with various concentrations of NK cells added back. Effector cells and Raji target cells were mixed and treated with anti-CD38 mAb, anti-CD3xCD20 bsAb or both mAb and bsAb ( figure 1).Results As expected, anti-CD38 mAb enhanced the killing of CD38+ Raji cells, and CD3×CD20 bsAb enhanced the killing of CD20+ Raji cells. When used alone, anti-CD38 mAb had little effect on CD20+ Raji cells, and CD3×CD20 bsAb had little effect on CD38+ Raji cells. However, when mAb and bsAb were combined and co-cultured with both CD20+ and CD38+ Raji, NK cells activated by anti-CD38 mAb enhanced CD3×CD20 bsAb-mediated T cell activation and enhanced T cell-mediated killing of CD20+ Raji cells. Similarly, T cells activated by CD3×CD20 bsAb enhanced NK cell activation and enhanced NK cell-mediated killing of CD38+ Raji cells. These data demonstrate that treatment with both mAb and bsAb results in ‘cross help’ between NK cells and T cells and enhances activation of both NK and T cells.Conclusions ‘Cross help’ between NK cells and T cells results in enhanced target cell killing when mAb and bsAb are given together. These data provide further support for evaluation of therapeutic strategies that concurrently engage NK and T cells through mAbs and bsAbs.References Weiner GJ, et al. The role of T cell activation in anti-CD3 x antitumor bispecific antibody therapy. J immunol. (Baltimore, Md.: 1950), 1994;152(5):2385-2392.Wang Z, et al. Bispecific antibody-activated T cells enhance NK cell-mediated antibody-dependent cellular cytotoxicity. J Hematol Oncol. 2021;14:1-4.Arora J, et al. T-cell help in the tumor microenvironment enhances rituximab-mediated NK-cell ADCC. Blood 2024;143(18):1816-1824.Abstract 703 Figure 1Cross-help between mAb-activated NK cells and bsAb-activated T cells. CD38+ Raji cells (left) and CD20+ Raji cells (right) were co-cultured with NK and T cells. mAb-activated NK cells enhanced T cell-mediated killing of CD20+ cells, and bsAb-activated T cells enhanced NK cell-mediated killing of CD38+ cells