In early pioneering studies on the immune status of melanoma SLN, Cochran and colleagues convincingly demonstrated that DC in SLN were more immune suppressed than DC in further downstream located TDLN [
22,
23]. This observation suggested DC to be a prime target of melanoma-induced immune suppression, consistent with their pivotal role in initiating T-cell-mediated anti-tumor immunity. Our group was the first to characterize and functionally test conventional DC (cDC) subsets, distinguishing migratory from LN-resident (LNDC) subsets, in human skin-draining LN using multiparameter flow cytometry [
24] closely followed by Segura and colleagues [
25]. We identified two migratory CD1a
+ cDC subsets, i.e., dermal DC (DDC) and Langerhans cells (LC), and two LNDC CD1a
− cDC subsets, distinguished by absence or presence of CD14 expression (see Table
1). The relative importance and varying roles of these cDC subsets in the priming of immune responses in healthy human LN remains largely elusive, but some clues are emerging. The migratory subsets take up antigens in the skin and will subsequently migrate to the skin-draining LN, where they can present those antigens to T cells. The two LNDC subsets are found in skin-draining LN but not among DC migrated from skin explants and are recruited from the peripheral blood to the LN [
26]. They are key players in cross presentation as evidenced by the high surface levels of cross-priming associated markers CLEC9A and BDCA3/CD141 (as well as expression of BATF3 mRNA; van de Ven et al., unpublished data) and by correlation of their frequencies to cross-presentation ability of melanoma SLN single-cell suspensions, which we observed after TLR9-mediated conditioning [
26]. Importantly, although the migratory subsets appeared more phenotypically mature under steady-state conditions, ex vivo isolated LNDC (both CD14
− and CD14
+) subsets proved more powerful in vitro primers of allogeneic effector T cells, which might tie in with higher release levels of T-cell-activating cytokines [
24]. Functional differentiation between the CD14
− and CD14
+ subsets remains obscure, but both may be involved in the priming of systemic anti-tumor effector T-cell responses, as we found the activation state of either to be associated with distant recurrence-free survival in early stage melanoma [
27]. Another DC subset in skin-draining LN are plasmacytoid DC (pDC) [
26]. These cells are poor antigen presenters, but are powerful producers of type I interferons upon TLR activation [
28]. As such, pDC play an important role in the activation of cDC and other immune cells.
Table 1
Conventional dendritic cell subsets found in skin-draining lymph nodes
Langerhans cells | CD1ahiCD11cint | Skin (migratory) | Primary tumor |
Dermal dendritic cells | CD1aintCD11chi | Skin (migratory) | Primary tumor |
CD14− LNDC | CD1a−CD11c+BDCA3hiCD14− | Circulation (LN resident) | LN metastasis |
CD14+ LNDC | CD1a−CD11c+BDCA3loCD14+ | Circulation (LN resident) | LN metastasis |
In melanoma, we observed a significant negative correlation between the activation state (based on CD83 expression) of DDC and LC in the SLN and primary tumor burden (Breslow thickness) [
27]. Interestingly, primary tumor burden was not shown to have a significant effect on either the frequency or activation state of LNDC subsets. However, the presence of SLN tumor metastases did have a significant impact on both the frequency and activation state of conventional LNDC, the latter showing a reverse correlation with the size of the metastasis (Table
1). This suggests that the primary melanoma can create a pre-metastatic niche in the TDLN by suppressing the activation states of migratory cDC subsets, which was shown to be associated with a shorter local recurrence-free survival. Subsequently, TDLN metastasis suppress LNDC which, interestingly, was shown to be associated with a worse distant recurrence-free survival [
27]. The latter indicates an essential role for conventional LNDC in the induction of effective systemic anti-tumor immunity.