Abstract Complement cleavage products generated during activation serve a wide range of functions from immune defense against microbes to homeostasis. However, the functions of complement activated products such as C4a, albeit present at high levels in lupus, remain unknown. Studying the functions of complement activated products is challenging. Studies using purified complement proteins can be confounded by minute contaminants with potent effects. Transgenic animals often provide more unequivocal insights. However, the generation of transgenic animals is highly involved and time-consuming, especially when backcrossing to genetically susceptible backgrounds is required for the development of diseases. Here, as an illustration for studying the functions of a complement activated product in vivo, we employed hydrodynamic injections to deliver expression plasmids encoding C4a to mouse liver. This technique allows hepatocytes to synthesize and directly secrete C4a into the circulation without disrupting intact C4. Using this system, we were able to detect exogenous C4a protein >2 months post-injection in the lupus prone MRL/lpr mice. Preliminary phenotyping on MRL/lpr mice overexpressing C4a revealed two immunologically relevant features: 1) the frequency of T cells was reduced and 2) the levels of autoantibodies against double-strand DNA were elevated comparing with vector controls. These data suggest that C4a can potentially modulate lupus manifestations. While detailed analyses are ongoing to determine the functions of C4a in lupus, our study illustrates that in vivooverexpression using hydrodynamic injections represents a relevant approach for studying the functions of complement activated products in disease models. supported by the Lupus Research Alliance