Fig. 4: In vivo pH imaging in subcutaneous EL4 lymphoma allows assessment of tumor heterogeneity.
a pH imaging in subcutaneously implanted EL4 lymphoma (white ROI) reveals heterogeneous tumor acidification. The white square indicates the native CSI resolution. b The majority of the tumor shows only one pH compartment being close to physiological pH conditions (d), indicated by one OMPD-C5-peak (blue triangle, spectrum from white circle ROI in b). c One or multiple subregions exhibit a second, acidic pH compartment, which is detectable as a second peak for the OMPD-C5-resonance (red triangle) in the spectrum (spectrum from white circle ROI in c) (e). Tumors generally show acidification of the mean pH (orange diamonds) compared to healthy muscle (blue squares) or blood pH (blue circles) (f), with its physiological compartment (blue triangle) being comparable to muscle pH, while the acidic pH compartment (red triangle) and the lowest single voxel mean pH (red crosses) are acidified by up to 0.6 pH units. Values are presented as means ± SD (n = 12 for muscle and blood vessel ROIs, n = 11 for tumor ROI-derived values, values are derived from independent probe injection experiments). g Histological analysis (n = 9 tumors) of the resected tumors confirms heterogeneity. h Quantification of immunohistochemical stainings of tumors in g are performed by classifying viable tumor cells (necrotic cells in black) into positive (red) and negative cells (blue). i Tumor areas bearing an acidic pH compartment tend to show a stronger expression of cleaved caspase 3 with increasing tumor acidification. Correlations for 2- (r = −0.35, p = 0.05) and 1-pH compartment regions (r = 0.14, p = 0.67) were assessed by linear regression.
