The present disclosure features the use of chimeric CD3 proteins to modulate T cell Receptor (TCR) signaling. Specifically, the disclosure is based, in part, on the discovery that chimeric CD3 proteins (e.g., CD3delta, CD3gamma, and CD3epsilon) having all or most of their extracellular domain fused to an antigen binding domain can activate the TCR in the presence of a cognate antigen. The disclosure is further based on the observation that the above chimeric proteins can be potentiated through the inclusion of a co-stimulatory domain in the intracellular portion of the chimeric molecule. Thus, the preferred elements of the engineered signaling complexes of the disclosure include an antigen binding domain, an extracellular domain derived from one of the above CD3 proteins, and an intracellular co-stimulatory domain.