Most solid tumors are heterogeneous and have an immunosuppressive tumor microenvironment (TME). These characteristics could be the reason why the therapeutic efficacy of T cell engagers (TCE) and chimeric antigen receptor (CAR)-T cell therapies in solid tumors is relatively limited compared to their performance in treating hematological malignancies. 

Tumor heterogeneity involves diverse subclones with distinct genomic alterations and antigenic profiles, making it difficult for targeted therapies to uniformly eliminate all cancer cells. Additionally, it is widely recognized that intrinsic heterogeneous antigen expression and antigen escape during treatment can contribute to limited outcomes of molecular targeting therapies.  

Meanwhile, the immunosuppressive environment is created by the recruitment of suppressive immune cells, which produce inhibitory factors including cytokines and immune checkpoint molecules. This complex ecosystem further hampers the immune system's ability to mount an effective response against solid tumors. 

Unlike immune checkpoint inhibitors like anti-PD-1/PD-L1 antibodies that can invigorate intrinsic antigen recognition system of T cells, T cell based-therapy such as TCE and CAR-T may be susceptible to tumor heterogeneity and the suppressive TME. Along with direct T-cell recruitment to tumor sites, the accomplishment of alleviating immunosuppressive TME or overcoming heterogeneity, or both, could be the rational strategy to enhance the efficacy of conventional TCE and CAR-T therapy. 

By understanding the intricate interplay between tumor cells, immune cells, and the TME, and by focusing on approaches that tackle tumor heterogeneity and/or improve the immunosuppressive environment, we can pave the way for the development of more effective immunotherapies for patients with solid tumors.