Dendritic cells (DC) are a complex group of mainly bone marrow derived cells that play an important role in the afferent branch of the immune response [1]. However, DC represents only a minute subpopulation of the peripheral blood mononuclear cells (PBMC), as well as of bulk cellular populations of the lung, intestine, genitourinary tissue, and lymphoid tissue. DC also has been found in the epidermis, dermis and mucous membranes and constituting about 2% of the total cellular population of the human epidermis [2, 3]. The Langerhans cells (LC) are a skin derived-DC, that have the capability to travel to the regional lymphoid organs after take up of antigen and undergo there an activation/maturation step. Thereafter, LC interacts and activates T cells. Because of such significant capability to take up soluble antigens, process and present them to responder cells in the lymphoid tissues in the context of the restricted MHC pathway, LC have been considered one of the most important elements in the afferent arm of the immune response [1–4].

Recent, successful efforts to generate DC from PBMC derived monocytes or from CD34 blood precursors by utilizing GM-CSF and IL-4, as well as GM-CSF and/or TNF, has enabled us to obtain PBMC derived DC (Mo-DC)) [5–8]. In addition, an approach has been developed to generate LC from isolated monocytes (Mo-LC) [9].

Despite the successful efforts in the generation of DC from blood precursors, the characterization of surface markers on human DC has been a very difficult and elusive task because the lack of appropriate reagents with high specificity for DC identification [4]. However, some molecules whose genes recently have been cloned and sequenced (e.g. CD83, DEC-205) have been found strongly associated with DC [5, 6]. In addition, a panel of monoclonal antibodies (e.g. CMRF-44) that recognize molecules on DC has been raised [7]. There is a growing need for cluster and establishment of a common and comprehensive nomenclature for such DC associated molecules, as well as to clarify and define the lineage(s) of DC and the existence of DC subsets. These developments have prompted the set up of diverse approaches that evaluated the reactivity of a group of mAb against populations of DC [10–29]. Therefore and within the scope of this study, we set up a flow cytometry approach and evaluated a panel of 20 mAb against two populations of DC aiming to determine the kinetics of expression of antigens on DC at diverse intervals of time and the likelihood to identify markers for DC subsets [12–29].

Flow cytometric assessments of monoclonal antibodies (mAb) reactivity against dendritic cells (DC) was undertaken by measurement of the % of reactivity (Figure 1,2,3,4) and mean fluorescent intensity (MFI) of two populations of DC at 7, 14, 21 and 28 days (Table 1). At day 7 (Figure 1), the flow cytometry evaluation showed that, antibodies 70011 (BG6), 70194 (HP-F1 (CD85i)), 70293 (BU10), 70511 (RFD-1) and 70375 (CMRF-44) recognized less than 20 % of Mo-DC. In contrast, 70345 (7H5 (CD85a)), 70358 (ZM3.8 (CD85j)), 70807 (CDlb/c), 70806 (55K-2 fascin), 70776 (MMR1.16 (CD206)), 70767 (MMR190.BB3 (CD206)) and 70772 (L25) reacted with more than 50% of Mo-DC. Moreover, 70345 (7H5 (CD85a)), 70358 (ZM3.8 (CD85j)), 70376 (CMRF-56), 70807 (CDlb/c), 70806 (55K-2 fascin), 70776 (MMR1.16 (CD206)), 70767 (MMR190.BB3 (CD206)) and 70772 (L25) also showed increase MFI reactivity against Mo-DC (Table 1).

The mAb 70011 (BG6), 70293 (BU10) and 70375 (CMRF-44) recognized less than 20% Mo-LC while 70511 reacted with only 21% of the cells. In contrast, 70194 (HP-F1 (CD85i)) showed that 87% of Mo-LCs were positive. Also, 70345 (7H5 (CD85a)), 70358 (ZM3.8 (CD85j)), 70512 (RFD-7), 70802 (MR15-2 (CD205)), 70807 (CDlb/c), 70806 (55K-2 fascin), 70776 (MMR1.16 (CD206)), 70767 (MMR190.BB3 (CD206)), 70772 (L25) reacted with more than 50% of Mo-LC. The increase in % of positive cells was also paralleled by MFI increases (Table 1).

At day 14 (Figure 2), 14 mAb recognized more than 50% of the Mo-DC, while 70427 (DCGM-4 (CD207)), 70451 (TPD153), 70511 (RFD-1), 70512 (RFD-7) and 70375 (CMRF-44) recognized between 20 and 50% of Mo-DC. 70011 (BG6) reacted with 7% of the Mo-DC. The reactivity of 70011 (BG6) against Mo-LC was indistinguishable from the background while the other nineteen mAb recognized more than 48% of Mo-LC. At day 21 and 28, all the mAb reacted with more than 20% of Mo-DC (Figure 3 and 4). Also, the mAb yielded a significant MFI with Mo-DC (Table 1). In addition, nineteen mAb yielded significant MFI with Mo-LC whilst only 70512 (RFD-7) did not. (Table 1) At day 21 (Figure 3), seventeen mAb reacted with more than 50% of the Mo-DC, only 70346 (42D1 (CD85f)), 70511 (RFD-1) and 70512 (RFD-7) were below 50%. In contrast, only seven mAb reacted with more than 50% Mo-LC but only 70512 (RFD-7) reacted with less than 20% Mo-LC. At day 28 (Figure 4), sixteen mAb reacted with more than 50% Mo-DC. The mAb 70346 (42D1 (CD85f)), 70511 (RFD-1), 70512 (RFD-7) and 70808 (DEC-205 (CD205)) were below 50% while 70375 (CMRF-44) reacted with less than 20% of Mo-LC. Seventeen mAb reacted with more than 50% of Mo-LC.

The simultaneous evaluation on Mo-DC and Mo-LC of FSC (flow cytometric assessment of size) and mAb reactivity at four time points demonstrated the presence of significant variability within the Mo-DC and Mo-LC populations (Figure 5). At day seven, there was several mAb that showed strong reactivity and can be grouped together (Figure 5 panel C mAb-MMR.190.BB3 and panel E mAb-L25). In addition there was another group of mAb with an intermediate level of reactivity that can be grouped together (Figure 5 panel D mAb-7H5 and panel F mAb-ZM-38). However, some mAb showed no reactivity at day seventh (Figure 5 panel A mAb-BG6) compared with the negative control (Figure 5 panel B-Second Ab). In addition, variations in the reactivity of the mAb at diverse time points were found. Moreover, at day seven it was found that mAb HP-F1 was negative for Mo-DC and positive for Mo-LC, thus discriminating Mo-DC from Mo-LC. Overall, this set of results demonstrates the existence of antigenic differences between Mo-DC and Mo-LC, even though there was only a single difference in the cytokine mixture utilized. Moreover, within the panel analyzed, it was found that at day 7, there is a mAb that showed reactivity only against Mo-DC with high FSC (mAb-TPD153) or against a fraction of Mo-DC (mAb-DC-LAMP, mAb-55K-2). These results point toward the existence of subsets within the Mo-DC and Mo-LC populations.

Of note, the studies on the reactivity of two set of mAb (mannose receptor and immunoglobulin-like transcript molecules) raised very interesting considerations about the role of these antigens in the functionality of DC.

This study showed that DC expressed significant amounts of two lectin-type receptors: the mannose receptor (MMR) detected by the mAbs MR 15-2 (CD206), MMR1.16 (CD206), and MMR190.BB3 (CD206?), and the antigen DEC-205 detected by the mAbs DEC-205 (CD205) and MMRI-4 (CD205). Both molecules belong to a family of transmembrane C-type lectins. Both molecules have (i) a cysteine-rich domain, (ii) a fibronectin type II domain, (iii) multiple C-type carbohydrate recognition-like domains, (iv) a transmembrane domain and (v) a short cytoplasmic tail [6, 12, 24, 25, 27]. Both molecules the MMR [24, 25, 27] and DEC-205 [6, 12], have been implicated in the uptake of carbohydrate-conjugated antigens by DC. MMR has been found on the cell surface of macrophages and its carbohydrate recognition domains mediate the endocytosis of (i) glycoconjugates containing mannose, (ii) fucose, (iii) acetylglucosamine, (iv) glucose residues, (v) microorganisms expressing mannose or acetylglucosamine on the surface. Notably, all these terminal sugars are neither common membrane components of mammalian cells nor common components of serum proteins. Therefore, it is feasible to consider that this two lectin-type receptors: the mannose receptor (MMR) and DEC-205 may be involved in the discrimination between self and non-self antigens as well and could contribute to enhance the capability of dendritic cells to generate primary T cell responses against infectious agents and soluble antigens that carry these types of carbohydrates.

In addition, this study provides the first comparison of the distribution of the MMR and DEC-205 on two human blood derived DC, Mo-LC and Mo-DC.

A panel of mAb that recognize several immunoglobulin-like transcript molecules (ILT) was also evaluated in this study. The mAb HP-F1 (CD85i) identified ILT2 [13], ZM3.8 (CD85j) recognized ILT3 [17], 42D1 (CD85f) identified ILT4 [16] and 7H5 (CD85a) recognized ILT5 [15]. One significant feature of the ILTs molecules is their capability to bind MHC class I molecules [13]. Furthermore, the molecules ILT2, ILT3, ILT4, and ILT5 have been considered to play a role as inhibitory receptors because they carry the immunoreceptor tyrosine-based inhibitory motif (ITIM) in the intracytoplasmic tail. Because of (i) the binding capability of ILT2-5 to MHC class I molecules [13, 17], (ii) the presence of inhibitory motifs within the cytoplasmic tail of ILT2-5 [13, 15–17] and (iii) the identification of ILT2-5, within diverse populations of dendritic cells [10, 17], is likely that the ILT molecules identified on DC may not only perform a key role as the receptor for the "missing self [30] but also could contribute to control the initial steps of activation of the immune response; Thus, this would tune up the fine balance between the activation signals provided by the simultaneous cross-linking of MHC class I or II and co-stimulatory molecules during the interaction between DC with T cells and the inhibitory signals mediated through the whole variety of ILTs molecules carrying ITIM motifs. Further functional studies of the role of ILTs on DC will provide insight toward the understanding of these mechanisms and its role in the regulation of the DC function.

The immunophenotyping assays described in this report enabled us to determine in human DC: (i) the existence of differences between Mo-DC and Mo-LC populations; (ii) the existence of subsets within the Mo-DC and Mo-LC populations; (iii) the kinetics of antigens expression at diverse intervals of time on DC; and (iv) specific markers for subpopulations of DC.