Antibiotic resistance urgently calls for the development of alternative therapies against bacterial infections. A promising strategy is to boost the host immune system through antibodies, either indirectly via vaccination or directly via therapeutic antibodies. However, real progress in such developments is hampered by our limited knowledge of the processes underlying antibody-dependent immune clearance.

 “What is a good antigen?” is a key question that many scientists in antibody therapeutics try to resolve. The bacterial cell is a highly complex landscape of hundreds of different antigens, and we don’t really understand which bacterial antigens are good targets for antibody therapy. One of the major mechanisms through which antibodies can kill bacteria is through leveraging complement activation upon binding to bacterial surface antigens. Activation of the complement cascade can lead to insertion of large pores into bacterial membranes and/or phagocytosis. However, the determinants for antibody-mediated bacterial killing via complement remain poorly understood. Therefore, there is a need for new tools to identify anti-bacterial antibodies and relate the antigenic target with the antibody’s functionality.

In this paper, we establish an approach to identify novel anti-bacterial antibodies from human B cells. Although it is common in the field to stain B cells with purified antigens, we decided to go for a more holistic approachand stain the B cells with entire living bacteria, thereby keeping antigens in the natural context of the complex bacterial membrane. Using this approach, we identified 29 novel antibodies against Klebsiella pneumoniae, an important cause of drug-resistant infections. Using genetic and functional approaches, we managed to couple the antibody’s functionality to its antigenic target.

 The main surprises of our study?
1) The antigen matters!

We found that the capacity of antibodies to activate complement depends on the antigenic target.

2) Mixing helps!

We discovered that some antibodies can act synergistically. When added as a mixture, some antibodies can strengthen each other’s binding to the bacterial surface. Contrary to our expectation, this cooperation between antibodies was independent of the antibody’s Fc-tail. 

In all, we anticipate that our approach will accelerate discovery of monoclonal antibodies against bacteria and stimulate others to explore whether antibody combinations are a promising route to develop potent antibody therapies.