CD47 is upregulated on circulating hematopoietic stem cells and leukemia cells to avoid phagocytosis

Abstract

Macrophages clear pathogens and damaged or aged cells from the blood stream via phagocytosis. Cell-surface CD47 interacts with its receptor on macrophages, SIRPalpha, to inhibit phagocytosis of normal, healthy cells. We find that mobilizing cytokines and inflammatory stimuli cause CD47 to be transiently upregulated on mouse hematopoietic stem cells (HSCs) and progenitors just prior to and during their migratory phase, and that the level of CD47 on these cells determines the probability that they are engulfed in vivo. CD47 is also constitutively upregulated on mouse and human myeloid leukemias, and overexpression of CD47 on a myeloid leukemia line increases its pathogenicity by allowing it to evade phagocytosis. We conclude that CD47 upregulation is an important mechanism that provides protection to normal HSCs during inflammation-mediated mobilization, and that leukemic progenitors co-opt this ability in order to evade macrophage killing.

Figure 1. Mobilization or inflammation induces CD47 up-regulation in hematopoietic stem and progenitor cells

(A) Expression level of CD47 on c-Kit+ cells is shown for day 2 Cy/G mobilized BM. (B) Myeloid progenitor and stem cell gates are shown for day 2 mobilized BM. Histograms on right show level of CD47 expression in marrow LT-HSC and GMP for steady-state (light gray shaded histogram), day 2 mobilized (black line), and day 5 mobilized (dark gray shaded histogram). (C) Relative MFI of CD47 for GMP on days 0-5 of Cy/G mobilization. Results were normalized so that steady state GMPs had MFI 100. (D) Myeloid progenitor and stem cell gates are shown for day 2 BM post-LPS treatment. Histograms show level of CD47 expression on day 2 post-LPS (black line), day 5 post-LPS (dark gray shaded histogram), steady state (light gray shaded histogram), and IAP-/- (black shaded histogram). (E) MFI for human CD47 on HSCs from human bone marrow (NBM), cord blood CB), or mobilized peripheral blood (MPB). (F) Evaluation of KLS cells in the hematopoetic organs of IAP+/+ and IAP-/- mice mobilized on days 2 through 5. Two mice are analyzed per genotype per day.

Figure 2. CD47 deficient HSCs are efficiently phagocytosed by macrophages but otherwise exhibit normal hematopoietic developmental potential

(A) Stem cells (left column) are gated on Lin- c-Kit+ Sca-1+ cells. Myeloid progenitors (right column) are gated on Lin- c-Kit+ Sca-1+ cells. Frequency in whole bone marrow is shown adjacent to each gated population. (B) Colony output on day 7 of individually sorted LT-HSC. G-granulocyte, M-macrophage, GM-granulocyte and macrophage, GEMM-granulocyte, macrophage, erythroid, and megakaryocyte, Meg- megakaryocyte. (C) Survival curve of recipient mice given lethal radiation and transplanted with the cells shown, n=5 for each group. (D) Examples of chimerism plots at 4 weeks post-transplant for IAP+/+ or IAP-/- donors. (E) Summary of chimerism analysis of mice transplanted with either 50 or 500 IAP+/+ or IAP-/- cells. (F) Results of phagocytosis assays using IAP+/+ or IAP-/- c-Kit enriched bone marrow. n=3, error bars represent 1 SD. (G) Photomicrographs of phagocytosis assays taken after 2 hours.

Figure 3. IAP+/- HSCs have a competitive disadvantage relative to wild-type HSCs due to macrophages

(A) MFI of CD47 on IAP+/+, IAP+/-, and IAP-/- LT-HSC. (B) Donor chimerism analysis for transplants of IAP+/+, GFP+ or IAP+/-, GFP+ marrow cells. Loss of donor chimerism is shown using Kaplan-Meier curves. (C) Experimental design for assessing effect of CD47 heterozygosity during LPS challenge. (D) Change in percent chimerism of host KLS cells compared to expected chimerism based on peripheral blood granulocytes is shown. Error bars represent 1 SD and p-values were obtained by ANOVA statistics. n=5 for IAP+/+ with clodronate (CLOD), n= 7 for IAP+/+ with control (CTRL), n=4 for IAP+/- with clodronate, n=7 for IAP+/- with control. (E) Results of in vivo phagocytosis assay comparing IAP+/+ and IAP+/- c-Kit enriched cells. Percent recipient GFP+ F4/80 cells is shown (n= 3 for each group, error bars represent 1 SD).

Figure 4. CD47 is up-regulated in murine and human myeloid leukemia

Typical stem and progenitor plots are shown for leukemic hMRP8bcrabl × hMRP8bcl2 cells compared to control non-leukemic animals. Lin- c-Kit+ Sca-1+ gated cells from control bone marrow (A) and leukemic spleen (B) and Lin- c-Kit+ Sca-1- gated cells from control bone marrow (C) and leukemic spleen (D). Frequency is shown as a percentage of entire marrow or spleen mononuclear fraction. (E) Quantitative RT-PCR for CD47. Data are shown from 3 sets of mice transplanted with either leukemic or control hRMP8bcrabl × hMRP8bcl2 BM cells and then sacrificed 2-6 weeks later. Results were normalized to beta-actin and 18S rRNA expression. Fold change relative to control transplanted whole Bcl-2+ BM cells was determined. Error bars represent 1 SD. (F) Histograms show expression of CD47 on gated populations for leukemic (gray) and control (black) mice. (G) Comparative FACS histograms of human CD47 expression by normal (red; n=6) and acute myelogenous leukemic (AML, blue; n=6) hematopoietic stem cells (HSC; CD34+CD38-CD90+Lin-) and progenitors (CD34+CD38+Lin-). (H) Comparative FACS histograms of CD47 expression by normal (red) and chronic myelogenous leukemia (blue) hematopoietic stem cells (HSC; CD34+CD38-CD90+Lin), committed progenitors (CD34+CD38+Lin-), and Lin+ cells.

Figure 5. Over-expression of murine CD47 rescues the growth defect of MOLM-13 myeloid leukemia cells

(A) GFP and human CD45 chimerism for mice transplanted with untransduced MOLM-13 cells (5×10⁵ and either 5×10⁵ Tet (n=6) or Tet-CD47 MOLM-13 (n=8) cells. (B) MOLM-13 chimerism in hematopoietic tissues was determined by human CD45 chimerism and measurement of tumour lesion size. (C) Hematoxylin and eosin sections of Tet-CD47 MOLM-13 transplanted liver (200×) (top panel). Periportal (arrow) and sinusoidal (arrowhead) tumor infilitration is evident. Examples of liver tumor formation and hepatomegaly in Tet-CD47 MOLM-13 transplanted mice versus control transplanted mice (middle panel). GFP fluorescence demonstrates tumor nodule formation as well diffuse infilitration (bottom panel). (D) 1×10⁶ Tet (n=5) or Tet-CD47 MOLM-13 (n=4) cells were injected into the right femur of RAG2-/-, Gc-/- mice and the tissues were analyzed 50-75 days later for chimerism of MOLM-13 cells in BM. (E) Survival curve of mice transplanted intrafemorally with Tet or Tet-CD47 MOLM-13 cells. (F) Representative FACS plots from mice transplanted intrafemorally with Tet or Tet-CD47 MOLM-13 cells. R-right, L-left.

Figure 6. Higher expression of CD47 on MOLM-13 cells causes increased tumorigenicity

(A) Histograms show CD47 expression in MOLM-13 high (black), MOLM-13 low (gray), and mouse bone marrow (shaded) cells, with MFI normalized for size shown. (B) The histograms show level of CD47 expression on CD47^hi MOLM-13 cells in untreated (shaded) and doxycycline treated (shaded) mice, with MFI normalized for size shown. (C) Survival of mice transplanted with 1 × 10⁶ CD47^hi, CD47^lo MOLM-13 cells, or CD47^hi MOLM-13 cells with doxycycline administration started after 2 weeks post-transplant. n=5 for CD47^hi and CD47^hi plus doxycycline. n=10 for CD47^lo. (D) Liver and spleen size of mice at necropsy or 75 days after transplant with 1 × 10⁶ CD47^hi, CD47^lo MOLM-13 cells, or CD47^hi MOLM-13 cells with doxycycline. (E) Bone marrow and spleen chimerism of human cells in mice at necropsy or 75 days after transplant with 1 × 10⁶ CD47^hi, CD47^lo MOLM-13 cells, or CD47^hi MOLM-13 cells with doxycycline. (F) Murine CD47 expression on CD47^lo MOLM-13 cells engrafting in bone marrow (open) compared with original cell line (shaded), with MFI normalized for size shown. (G) Bone marrow and (H) liver histograms are shown for MOLM-13 cells in mice 35 days after transplant with 1 × 10⁶ CD47^lo MOLM-13 cells. Parental Tet MOLM-13 (dark gray shaded) and Tet CD47^lo MOLM-13 CD47 (light gray shaded) expression, as well as engrafting cells in the clodronate (gray line) and control (black line) cohorts is displayed in the histograms. (I) MFI of CD47 channel in engrafting MOLM-13 cells in clodronate (CLOD) or control (CTRL) treated mice is shown for bone marrow and liver. Error bars represent 1 SD.

Figure 7. Increasing CD47 expression on MOLM-13 cells confers increasing ability to evade macrophage phagocytosis

(A) Tet, Tet-CD47^lo, Tet-CD47 bulk, or Tet CD47^hi MOLM-13 cells were incubated with bone marrow derived macrophages (BMDM) for 2 hours and phagocytic index was determined. Error bars represent 1 SD (n=6 for each time point). (B) Photomicrographs of BMDMs incubated with Tet or Tet-CD47 MOLM-13 cells at 2 and 24 hours (400×). (C) CD47^hi GFP and CD47^lo MOLM-13 RFP cells were co-incubated with BMDMs for 2 hours. Phagocytic index is shown for three separate samples for CD47^hi GFP (green) and CD47^lo MOLM-13 RFP (red) cells. (D) Photomicrographs show brightfield (top left), RFP (top right), GFP (bottom left), and merged (bottom right) images of CD47^hi GFP and CD47^lo MOLM-13 RFP cells were co-incubated with BMDMs for 24 hours.