The Role of CD4+ T Cells in a Murine Model of Acute Lymphoblastic Leukaemia.

To assess CD4-mediated anti-tumor immunity in a murine acute lymphoblastic leukaemia model system, we have generated a series of BCR-ABL positive pre-B cell lines expressing the surrogate tumor antigen ovalbumine. Upon intravenous injection, PKH-26-labeled leukaemia cells were taken up by splenic CD8+ but not by CD8− dendritic cells (DC). In comparison to PBS-injected DCs, CD8+ DCs also showed increased expression of CD40, CD80, and CD86. Purified DCs from leukemic mice stimulated transgenic DO11.10 T cells recognizing OVA^323–339 in the context of I-A^d, suggesting efficient presentation of the surrogate tumor antigen. Next, we utilized adoptive transfer of DO11.10 T cells to measure tumor-specific T cell responses in vivo. OVA-expressing BM185 cells were unable to directly stimulate DO11.10 T cells, as shown by ³H-thymidine incorporation. In contrast, DO11.10 T cells were activated in vivo in spleen and lymph nodes, as shown by upregulation of CD44 and CFSE staining, suggesting that DC effectively present tumor antigens to DO11.10 T cells in vivo. However, despite of detectable T cell activation and proliferative T cell responses, all animals succumbed to progressive leukemia. Furthermore, adoptive transfer of naïve DO11.10 cells did not induce protective anti-leukemia immunity. Interestingly, in vivo primed DO11.10 T cells did not express interferon-γ. We therefore hypothesized that inefficient in vivo priming of TH1 cells may contribute to immune evasion of ALL cells. To address this question, we primed DO11.10 T cells in vitro prior to adoptive transfer. In this setting, DO11.10 T cells expressed interferon-γ and induced regression of preestablished leukaemia. This effect was dependent on CD8 cells, as shown by in vivo depletion experiments. Our experimental system supports the concept of CD4-dependent antitumor immunity and provides a platform to assess immunological mechanisms of novel strategies to therapeutically enhance antileukaemic immune responses.