Monocyte–macrophage polarization and recruitment pathways in the tumour microenvironment of B‐cell acute lymphoblastic leukaemia

Abstract

Summary B‐cell acute lymphoblastic leukaemia (B‐ALL) reprograms the surrounding bone marrow (BM) stroma to create a leukaemia‐supportive niche. To elucidate the contribution of immune cells to the leukaemic microenvironment, we investigated the involvement of monocyte/macrophage compartments, as well as several recruitment pathways in B‐ALL development. Immunohistochemistry analyses showed that CD68‐expressing macrophages were increased in leukaemic BM biopsies, compared to controls and predominantly expressed the M2‐like markers CD163 and CD206. Furthermore, the "non‐classical" CD14+CD16++ monocyte subset, expressing high CX3CR1 levels, was significantly increased in B‐ALL patients' peripheral blood. CX3CL1 was shown to be significantly upregulated in leukaemic BM plasma, thus providing an altered migratory pathway possibly guiding NC monocyte recruitment into the BM. Additionally, the monocyte/macrophage chemoattractant chemokine ligand 2 (CCL2) strongly increased in leukaemic BM plasma, possibly because of the interaction of leukaemic cells with mesenchymal stromal cells and vascular cells and due to a stimulatory effect of leukaemia‐related inflammatory mediators. C5a, a macrophage chemoattractant and M2‐polarizing factor, further appeared to be upregulated in the leukaemic BM, possibly as an effect of PTX3 decrease, that could unleash complement cascade activation. Overall, deregulated monocyte/macrophage compartments are part of the extensive BM microenvironment remodelling at B‐ALL diagnosis and could represent valuable targets for novel treatments to be coupled with classical chemotherapy.

The Bone Marrow Niche in B-Cell Acute Lymphoblastic Leukemia: The Role of Microenvironment from Pre-Leukemia to Overt Leukemia

“…Interestingly, the delivery in this mouse model of anti-CSF1R antibodies, which depletes monocytes by inducing apoptosis and blocking their differentiation to macrophages, along with the tyrosine kinase inhibitor nilotinib, significantly increased the overall survival of leukemia-transplanted mice [ 111 ]. In line with their findings, our group confirmed the increase in NC CD14dimCD16+ monocytes in the PB of B-ALL patients at diagnosis, compared to controls, and highlighted that they express low levels of the CCL2 receptor CCR2, but very high levels of the Fractalkine (CX3CL1) receptor CX3CR1 [ 70 ], as previously shown for this monocyte subset under physiological conditions [ 112 ]. Interestingly, we also demonstrated for the first time a significant increase in CX3CL1 in the BM plasma of pediatric patients at B-ALL diagnosis compared to age-matched controls, suggesting that the CX3CL1/CX3CR1 could represent a novel chemokine axis crucial for the recruitment of NC monocytes to the leukemic BM niche [ 70 ].…”

“…In line with their findings, our group confirmed the increase in NC CD14dimCD16+ monocytes in the PB of B-ALL patients at diagnosis, compared to controls, and highlighted that they express low levels of the CCL2 receptor CCR2, but very high levels of the Fractalkine (CX3CL1) receptor CX3CR1 [ 70 ], as previously shown for this monocyte subset under physiological conditions [ 112 ]. Interestingly, we also demonstrated for the first time a significant increase in CX3CL1 in the BM plasma of pediatric patients at B-ALL diagnosis compared to age-matched controls, suggesting that the CX3CL1/CX3CR1 could represent a novel chemokine axis crucial for the recruitment of NC monocytes to the leukemic BM niche [ 70 ]. Importantly, CX3CL1/CX3CR1 has been already described as a deregulated pathway in the context of several tumors such as CLL and multiple myeloma, with a crucial role in the cross-talk between cancer cells and tumor microenvironment [ 113 , 114 ].…”

“…Beyond the CXCR4-CXCL12 axis, other chemokine pathways resulted significantly altered in the leukemic BM niche. Ex vivo experiments from different research groups indicate that leukemia stimulation of primary BM-MSCs is able to specifically upregulate the expression of CCL2, CXCL10, CCL22, CXCL8, and CXCL1 [ 67 , 68 , 69 , 70 ]. Chemokine level analyses in the BM plasma of independent cohorts of B-ALL patients and healthy donors (HDs) further validated some of these results, as CCL2, CXCL1 and CXCL8 were significantly increased in leukemic patients at disease diagnosis and relapse but were comparable to HDs at disease remission [ 62 , 67 , 70 ].…”

“…Ex vivo experiments from different research groups indicate that leukemia stimulation of primary BM-MSCs is able to specifically upregulate the expression of CCL2, CXCL10, CCL22, CXCL8, and CXCL1 [ 67 , 68 , 69 , 70 ]. Chemokine level analyses in the BM plasma of independent cohorts of B-ALL patients and healthy donors (HDs) further validated some of these results, as CCL2, CXCL1 and CXCL8 were significantly increased in leukemic patients at disease diagnosis and relapse but were comparable to HDs at disease remission [ 62 , 67 , 70 ]. Interestingly, these chemokines, abundant in leukemia-conditioned MSC supernatants, were able to potently attract B-ALL cells that, besides CXCR4 [ 63 , 71 ], showed high membrane levels of CCR4 (CCL22 and CCL2 receptor) [ 62 ] and variable levels of CXCR1/2 (CXCL1 and CXCL8 receptors) [ 72 ].…”

“…Concerning the ability of B-ALL cells to induce changes in the secretome of vascular cells, we recently demonstrated [ 70 ] that B-ALL cells upon co-culture with human umbelical vein endothelial cells (HUVEC) are able to significantly increase their secretion of CCL2, whose role in the B-ALL niche has been already highlighted. Moreover, we also demonstrated that inflammatory cytokines, abundant in the B-ALL BM niche, can impact chemokine production by vascular cells, such as, for example, increasing the release of CCL2 and CX3CL1 by HUVEC cells [ 70 ]. The fact that leukemic cells could modulate chemokine production by vascular cells is particularly intriguing in view of the key role of blood vessels in the recirculation of immune cells to the BM niche, where they can become dangerous allies of leukemic cells, as discussed in the following paragraph.…”