Immune cell identity behind the K^trans mapping of mouse glioblastoma

Dynamic contrast-enhanced MR imaging (DCE-MRI) can assess the integrity of the blood brain barrier (BBB) and has been used in GBM patients to determine glioma grade, predict prognosis, evaluate treatment response, and differentiate treatment-induced effect from recurrence. The volume transfer constant K^trans is the most frequently used metric in tumor assessment. Based on previous studies that a higher WHO grade of brain tumor was associated with greater impairments of immunity and that K^trans value was associated with the pathological grading, the relationship between differential composition of immune cells in GBM tissue and dynamic changes in K^trans mapping was anticipated in this study. The present study utilized an orthotopic allograft model of GBM in which mouse GL26 cells are implanted into Ccr2^RFP/wtCx3cr1^GFP/wt mice on a C57 background. The brain tumors exhibited heterogenous K^trans values with the coefficients of variation (CV) above 75%, or relatively homogeneous K^trans maps with CV values below 50%. The K^trans values of homogeneous tumors ranged between 0.02/min-0.32/min with a median value of 0.10/min. The immune cell composition defined by quantitative immunohistochemistry and cell sorting was compared between the tumors with K^trans values above 0.10/min (higher K^trans) or below 0.10/min (lower K^trans). Histological analysis showed that tumors with higher K^trans values exhibited greater numbers of CCR2^pos cells (257.60 ± 16.42/mm² vs 203.23 ± 12.20/mm², p = 0.04) and an increased ratio of CCR2^pos cells to CX3CR1^pos cells (1.20 ± 0.02 vs 0.38 ± 0.04, p = 0.001), the numbers of CX3CR1^pos cells did not differ significantly based on K^trans values (219.70 ± 16.20/mm² vs 250.38 ± 21.20/mm², p = 0.19). Flowcytometry analysis showed that tumors with higher K^trans values (above 0.1/min) were associated with greater numbers of both overall monocytes (54.93 ± 6.81% vs 29.75 ± 3.54%, p = 0.01) and inflammatory monocytes (72.38 ± 1.49% vs 59.52 ± 2.44%, p = 0.001). In contrast, tumors with lower K^trans values (below 0.1/min) exhibited greater numbers of patrolling monocytes (75.65 ± 4.14% vs 63 ± 6.94%, p = 0.05). In the tumors with lower K^trans values, all three types of tumor associated cells, including patrolling monocytes, inflammatory monocytes, and microglia cells possessed a higher proportion of cells at pro-inflammatory status (41.77 ± 6.13% vs 25.06 ± 6.72%, p = 0.05; 27.50 ± 2.11% vs 20.62 ± 1.87%, p = 0.03; and 55.80 ± 9.88% vs 31.12 ± 7.31%, p = 0.05), inflammatory monocytes showed fewer anti-inflammatory cells (1.25 ± 0.62% vs 3.16 ± 3.56%, p = 0.04). Taken together, differences in K^trans values were associated with differential immune cell phenotypes and polarizations. K^trans mapping may therefore represent a novel approach for defining the immune status of GBM.