Bacteria-instructed B cells cross-prime naïve CD8+ T cells triggering effective cytotoxic responses.

In addition to triggering humoral responses, conventional B cells have been described in vitro to cross-present exogenous antigens activating naïve CD8+ T cells. Nevertheless, the way B cells capture these exogenous antigens and the physiological roles of B cell-mediated cross-presentation remain poorly explored. Here, we show that B cells capture bacteria by trans-phagocytosis from previously infected dendritic cells (DC) when they are in close contact. Bacterial encounter "instructs" the B cells to acquire antigen cross-presentation abilities, in a process that involves autophagy. Bacteria-instructed B cells, henceforth referred to as BacB cells, rapidly degrade phagocytosed bacteria, process bacterial antigens and cross-prime naïve CD8+ T cells which differentiate into specific cytotoxic cells that efficiently control bacterial infections. Moreover, a proof-of-concept experiment shows that BacB cells that have captured bacteria expressing tumor antigens could be useful as novel cellular immunotherapies against cancer.

From cellular microbiology to bacteria-based next generations of cancer immunotherapies.

Pioneer work by Prof. Cossart among others, studying the interactions between pathogenic bacteria and host cells (this discipline was termed Cellular Microbiology), was fundamental to determine the bacterial infection processes and to improve our knowledge of different cellular mechanisms. The study of bacteria-host interactions also involves in vivo host immune responses, which can be manipulated by bacteria, being these last potent tools for different immunotherapies. During the last years, tumour immunotherapies, mainly the use of antibodies that target immune checkpoints [checkpoint inhibitors (CPI)], have been a revolution in oncology, allowing the treatment of tumours otherwise with very bad prognosis. In the same direction, bacteria inoculations have been used from long to treat some cancers; for example, non-muscle-invasive bladder cancer can be successfully treated with the bacterium Bacillus Calmette Guerin (BCG). More recently, it has been shown that microbiota could determine the success of CPI immunotherapies and intense research is being performed in order to use bacteria as immunotherapy tools due to their ability to activate the immune system. In this context, to expand the knowledge of the bacteria-immune system interactions will be fundamental to improve tumour immunotherapies.

T Cells Capture Bacteria by Transinfection from Dendritic Cells.

Recently, we have shown, contrary to what is described, that CD4(+) T cells, the paradigm of adaptive immune cells, capture bacteria from infected dendritic cells (DCs) by a process called transinfection. Here, we describe the analysis of the transinfection process, which occurs during the course of antigen presentation. This process was unveiled by using CD4(+) T cells from transgenic OTII mice, which bear a T cell receptor (TCR) specific for a peptide of ovoalbumin (OVAp), which therefore can form stable immune complexes with infected dendritic cells loaded with this specific OVAp. The dynamics of green fluorescent protein (GFP)-expressing bacteria during DC-T cell transmission can be monitored by live-cell imaging and the quantification of bacterial transinfection can be performed by flow cytometry. In addition, transinfection can be quantified by a more sensitive method based in the use of gentamicin, a non-permeable aminoglycoside antibiotic killing extracellular bacteria but not intracellular ones. This classical method has been used previously in microbiology to study the efficiency of bacterial infections. We hereby explain the protocol of the complete process, from the isolation of the primary cells to the quantification of transinfection.