a. An overview of unconventional and semi-professional APCs

As mentioned previously, cDCs can be considered as professional APCs. They are able to internalize exogenous antigens, constitutively express the antigen processing machinery and MHC-II, and express co-stimulatory molecules upon activation, and their main function is to initiate T cell responses (Kambayashi and Laufer, 2014).

Nonetheless, several other APC type have been described as being able to cross-present via MHC-I and/or present via MHC-II antigens to CD8⁺ and/or CD4⁺ T cells, as well as to present endogenously-expressed self-antigens through MHC-I or MHC-II, including cells from the hematopoietic and non-hematopoietic compartments (Kambayashi and Laufer, 2014). We refer to these cells as “unconventional” APCs in this manuscript. A major difference with professional APCs is that they do not constitutively express MHC-II but its expression is upregulated upon maturation under certain conditions.

The term “unconventional” has been used by others to describe certain types of APCs (Crispe, 2011; Reynoso and Turley, 2009; Uto et al., 2018; Valitutti and Espinosa, 2010). Other terms used in the literature include “atypical” (Kambayashi and Laufer, 2014), “non-classical” (Costantino et al., 2012), “non-traditional” (LaSalle et al., 1992), “non-conventional” (Horst et al., 2016) and

“alternative” (Moser and Brandes, 2006). Other terms, in opposition/reference to “professional”

have been also used, such as “non-professional” (Lopes Pinheiro et al., 2016; Mehrfeld et al., 2018; Montealegre and van Endert, 2018; Pisapia et al., 2015; Wosen et al., 2018), “amateur”

(Royer et al., 2018; Schuijs et al., 2019; Sprent, 1995), and “semi-professional” (Razakandrainibe et al., 2012).

The term “unconventional” is defined as “not based on or conforming to what is generally done or believed” (Oxford dictionary) and is used in this thesis to describe APCs that are not professional APCs and for which the most studied function in the literature is not related to antigen presentation. “Unconventional” is a very subjective, and somehow arbitrary term, which, in the context of research highly depends on the chronology of scientific discoveries, the proportion of scientists working on a given topic compared to another (e.g. study about innate vs adaptive functions of pDCs), and the dogma in place at a given moment in time. In addition, it could be related to the ontogeny of cell types.

21

In their review, Kambayashi and Laufer asked whether any cell could replace professional APCs (Kambayashi and Laufer, 2014). Some studies have shown that DCs were necessary and sufficient to activate naïve T cells, while other studies showed that, under certain conditions, DCs were not required for the development of CD4⁺ T cell responses (Kambayashi and Laufer, 2014).

Depending on the cell type, the location, and the immunological context, unconventional APCs play various roles in immunity and tolerance (Duraes et al., 2013; Kambayashi and Laufer, 2014;

Mehrfeld et al., 2018; Reynoso and Turley, 2009).

Hematopoietic cells

Mast cells, eosinophils and basophils have been described to express MHC-II under certain conditions and behave as APCs, although it is debated in the literature (Kambayashi and Laufer, 2014; Mehrfeld et al., 2018). Most of the studies were performed in vitro and there is no evidence that these cells can present in vivo. In addition, neutrophils have been reported to perform MHC-I-mediated cross-presentation or MHC-IMHC-I-mediated presentation, inducing Th1 or Th17 polarization (Abi Abdallah et al., 2011; Beauvillain et al., 2007; Kambayashi and Laufer, 2014;

Mehrfeld et al., 2018; Vono et al., 2017). Results are controversial and here also, most of the studies were carried out in vitro.

In addition to their ability to differentiate into macrophages or inflammatory DCs, monocytes can migrate from the blood to LNs, acquire the expression of MHC-II, and present antigens as APCs per se (Jakubzick et al., 2017).

Furthermore, innate lymphoid cells (ILCs) have been implicated in MHC-II-mediated antigen presentation to CD4⁺ T cells and drive Th2 cell responses (Kambayashi and Laufer, 2014;

Mirchandani et al., 2014; Oliphant et al., 2014). Human CD4⁺ T cells themselves express MHC-II upon activation and present self-antigens to other CD4⁺ T cells (Costantino et al., 2012;

Kambayashi and Laufer, 2014; LaSalle et al., 1992).

Finally, megakaryocytes, the hematopoietic cells responsible for the maintenance of platelets, have been shown to uptake exogenous antigens, process and cross-present them to CD8⁺ T cells (Zufferey et al., 2017).

Non-hematopoietic cells

Several cells from the non-hematopoietic compartment have been shown to perform MHC-I-mediated cross-presentation or MHC-II-MHC-I-mediated antigen presentation; hepatocytes, epithelial and endothelial cells, originating from different organs such as liver, skin, LNs and parenchymal

22

tissues (Duraes et al., 2013; Kambayashi and Laufer, 2014; Mehrfeld et al., 2018; Reynoso and Turley, 2009).

Epithelial cells, including cells from the gastrointestinal tract, respiratory tract and cornea have been shown to present antigens through MHC-II (Kambayashi and Laufer, 2014; Royer et al., 2018; Wosen et al., 2018). These cells also include the APC type TECs, described in the “central tolerance” section (Anderson and Takahama, 2012; Klein et al., 2009; Roche and Furuta, 2015).

Endothelial cells from many tissues and organs have also been implicated in antigen presentation, with consequences in various pathologies, such as transplantation and parasite infection (Al-Soudi et al., 2017; Kambayashi and Laufer, 2014; Pober et al., 2017; Razakandrainibe et al., 2012;

Taflin et al., 2011).

Antigen-presenting non-hematopoietic cells also include LNSCs, among which LECs, described later in this introduction (Duraes et al., 2013; Reynoso and Turley, 2009; Turley et al., 2010).

Finally, aberrant acquisition of constitutive MHC-II expression has been reported in certain non-hematopoietic tumor cells, such as melanoma cells, and subsequent antigen presentation to CD4⁺ T cells (Donia et al., 2015).

Cross-dressing

In addition to the presentation of antigens internalized or endogenously-expressed, cells (APCs or not) can acquire MHC/peptide complexes from other cells, without the need to internalize and process antigens, a phenomenon called cross-dressing (Nakayama, 2014). Transfers occur via direct cell-cell contact, called trogocytosis, or through secretion of membrane vesicles, such as exosomes, by the donor cell (Nakayama, 2014). Donor cells can be APCs or not, including infected and tumor cells, and recipient cells also include APCs, unconventional APCs and non-APCs (de Heusch et al., 2007; Dubrot et al., 2014; Kedl et al., 2017; Koble and Kyewski, 2009;

Nakayama, 2014; Rouhani et al., 2015; Tamburini et al., 2014; Wakim and Bevan, 2011; Zhang et al., 2008).

This phenomenon, reviewed by Nakayama, give recipient cells the ability to present MHC-I- or MHC-II-restricted antigens, while not necessarily able to internalize and/or process those antigens, or which do not have the intrinsic ability to express MHC-II (Nakayama, 2014).

Plasmacytoid DCs and LECs as unconventional APCs

In this thesis, we consider pDCs and LECs as unconventional APCs, although these two distinct cell types cannot be considered on the same level regarding their properties to function as bona

23

fide APCs. For instance, in contrast with pDCs, LECs originate from the non-haematopietic compartment, they do not possess the ability to migrate, and they do not have the ability to upregulate the expression of co-stimulatory molecules, to cite a few examples (Humbert et al, 2016).

Therefore, whether a cell type could be considered as “unconventional” APC, as opposed to

“professional” APC, might depend on a spectrum rather than on a “all or nothing” concept. This spectrum could:

- start from cells such as LECs, which arise from the non-haematopoietic compartment (ontogeny), are well known to play structural roles (main function), whose role as APCs was recently discovered (chronology of discoveries), and which are not able to express molecules considered important for professional APCs (attributes), such as co-stimulatory molecules

- include pDCs, which, under certain circumstances (activation with specific stimuli) exhibit features that are not far from that of professional APCs (cDCs), regarding their antigen-presenting capacities.

- and end with cDCs, the “professional” APCs, whose main function is to initiate T cell responses.

Whether pDCs can be considered as “unconventional” APCs, as it is not a consensus, is discussed at the end of the description of antigen-presenting functions of pDCs, in the next section.