This pro-proliferative sign is not specific to pDCs, as mesenchymal stem cells (MSCs) are another cell type that support multiple myeloma proliferation directly via the suppression of T cells through the PD-1 axis, and others such as the DKK1+ MM cells and Il-6-producing undifferentiated MSCs [26,27,28]. This evidence concerns the gene DKK1 and AL amyloidosis.