Complement is a complex protein network of plasma, and an integral part of the innate immune system. Complement activation results in the rapid clearance of bacteria by immune cells, and direct bacterial killing via large pore-forming complexes. Here we review important recent discoveries in the complement field, focusing on interactions relevant for the defense against bacteria. Understanding the molecular interplay between complement and bacteria is of great importance for future therapies for infectious and inflammatory diseases. Antibodies that support complement-dependent bacterial killing are of interest for the development of alternative therapies to treat infections with antibiotic-resistant bacteria. Furthermore, a variety of novel therapeutic complement inhibitors have been developed to prevent unwanted complement activation in autoimmune inflammatory diseases. A better understanding of how such inhibitors may increase the risk of bacterial infections is essential if such therapies are to be successful.

M.L.A.); a.dimaggio@students.uu.nl (A.D.M.); Y.Yu@uu.nl (Y.Y.)Abstract: Nanobody-targeted photodynamic therapy (NB-PDT) has been recently developed as a more tumor-selective approach rather than conventional photodynamic therapy (PDT). NB-PDT uses nanobodies that bind to tumor cells with high affinity, to selectively deliver a photosensitizer, i.e., a chemical which becomes cytotoxic when excited with light of a particular wavelength. Conventional PDT has been reported to be able to induce immunogenic cell death, characterized by the exposure/release of damage-associated molecular patterns (DAMPs) from dying cells, which can lead to antitumor immunity. We explored this aspect in the context of NB-PDT, targeting the epidermal growth factor receptor (EGFR), using high and moderate EGFR-expressing cells. Here we report that, after NB-PDT, the cytoplasmic DAMP HSP70 was detected on the cell membrane of tumor cells and the nuclear DAMP HMGB1 was found in the cell cytoplasm. Furthermore, it was shown that NB-PDT induced the release of the DAMPs HSP70 and ATP, as well as the pro-inflammatory cytokines IL-1β and IL-6. Conditioned medium from high EGFR-expressing tumor cells treated with NB-PDT led to the maturation of human dendritic cells, as indicated by the upregulation of CD86 and MHC II on their cell surface, and the increased release of IL-12p40 and IL-1β. Subsequently, these dendritic cells induced CD4+ T cell proliferation, accompanied by IFNγ release. Altogether, the initial steps reported here point towards the potential of NB-PDT to stimulate the immune system, thus giving this selective-local therapy a systemic reach.Despite being a selective and controllable treatment against tumor cells, therapies known to be considerably aggressive towards all tissues (i.e., chemo-and radiotherapy) are still the mainstream clinical practice, when it comes to combat cancer [5]. Some current barriers for the establishment of PDT in routine practice are the required technology, complex dosimetry, limited tissue penetration of light and/or PS, lack of tumor specificity and prolonged skin photosensitivity [1,5]. Efforts have been put into improving the selectivity of PDT towards the cancer cells by using antibodies, an approach which is now in phase II clinical trial (NCT02422979) [6]. Conjugation of a water-soluble PS (IRDye700DX) to an antibody allows the PS to be specifically delivered to cancer cells overexpressing a certain antigen on the cell membrane. In this manner, two levels of specificity are established: the delivery of PS to target-expressing cells and the local application of light at the tumor site.Another strategy to render PDT more tumor-selective is nanobody-targeted PDT (NB-PDT), which also utilizes the near-infrared PS IRDye700DX. In this case, however, a nanobody is used to provide the first level of specificity [7,8]. NBs are the smallest binding domains found in nature (~15 kDa) and consist uniquely of the variable domain of heavy chain antibodies present in Camelids [9]. The use of a NB for PDT brings a ...