CCL21 and Dendritic Cell Suppression in Paediatric B Cell Acute Lymphoblastic Leukaemia

Despite the accumulation of genetic mutations likely to generate antigenic neoepitopes, acute lymphoblastic leukaemia (ALL) cells are poor at eliciting anti-tumour immune responses. Similarly to dendritic cells (DCs), B cells have antigen presenting capacity. In B cell ALL (B-ALL), blast expression of co-stimulatory molecules is down regulated whilst endogenous (tumour) antigens are presented through MHC class I and II[1], thus mimicking the DC peripheral tolerance mechanism. This leads to antigen specific anergy in naïve T cells capable of recognising the malignant cells and an anti-tumour immune response is not mounted. A robust DC response, presenting tumour antigens alongside co-stimulation, should be sufficient to overcome this since no intrinsic T cell defect exists in these patients. However, several studies have reported deficiencies in DC number and function in patients with B-ALL. Maecker et al. demonstrated that paediatric patients with B-ALL have significantly reduced blood DC counts at diagnosis, a reduction not attributable to disease induced bone marrow hypoplasia since blood DC numbers do not correlate with granulocyte or monocyte numbers or haemoglobin[2]. Mami et al. identified that CD34+ peripheral blood mononuclear cells from patients with B-ALL at diagnosis are ineffective at generating functional DCs in vitro suggesting that B-ALL cells, or leukaemia derived factors, may contribute to a block in DC development[3]. In this study, we monitored DCs and plasma cytokines in paediatric patients with B-ALL at diagnosis and throughout remission induction chemotherapy, in peripheral blood (PB) and the bone marrow (BM) tumour microenvironment, to identify on treatment changes in DCs and DC regulatory cytokines. 17 paediatric patients diagnosed with precursor B cell ALL were enrolled in this study. Matched PB and BM aspirate samples were collected at time points co-ordinating with treatment protocol (diagnosis, and induction days 8 and/or 15 and 29). Mononuclear cells and plasma samples were obtained by density gradient centrifugation. DCs were identified by flow cytometry (based on expression of HLA-DR(+), lack of B, T, NK and monocyte markers (CD3-, CD19-, CD56-, CD14-)). Plasma cytokines were analysed by Luminex multi-plex immunoassay. Our study confirmed that blood DCs are low in paediatric patients with B-ALL at diagnosis and that this scenario is mirrored in the BM tumour microenvironment. PB and BM DCs remained low through days 8 and 15 o