Tumour growth parameters of 4T1 tumour-bearing mice treated with histamine. a Experimental design. BALB/c mice were orthotopically inoculated with 4T1 cells and 2 weeks later were treated or not with 1 or 5 mg kg−1 of histamine. b The means ± SEM for the ratio between treated and untreated mice for tumour volume and weight at the end of the experimental period are shown (T test, *P < 0.05 with respect to untreated mice). c Effect of ionising radiation treatment (three doses of 2 Gy) on untreated and 1 mg kg−1 histamine-treated mice bearing 4T1 tumours. Tumour growth curves show the relative tumour volume for irradiated control and histamine-treated mice. Inset: Tumour weight at the end of the experimental period, relative to control non-irradiated group (T test, *P < 0.05). d–g Tumour sections from control (n = 10) and 5 mg kg−1 histamine-treated mice (n = 10) were stained with H&E or immunostained with specific antibodies. The box plots represent the median and interquartile range for each experimental group (Mann–Whitney test, *P < 0.05, **P < 0.01). d Mitotic index: number of cells with visible chromosomes at ×400 magnification in 5 random fields. e Percentage of PCNA-positive cells per field at ×400 magnification in 10 random fields. f # of vessels: Number of intratumoural vessels at ×200 magnification in 10 random fields (hotspots). g Number of TUNEL-positive cells per field at ×400 magnification in 10 random fields. h Number of microscopic metastatic foci covering lungs at ×400 magnification in 10 random fields. Box plots represent the median and interquartile range for each experimental group (Mann–Whitney test). i Representative images for H&E staining, TUNEL-positive cells and PCNA, histamine and HDC-positive immunostaining of paraffin-embedded 4T1 tumours (×630 and ×400 original magnification, scale bar = 20 µm). j Tumour sections from control and 5 mg kg−1 histamine-treated mice were stained with specific antibodies against histamine (HA) or histidine decarboxylase (HDC). The results representing the means ± SEM of the percentage of positive cells per field at ×400 magnification in 10 random fields are shown (t test). # number of polymorphonuclear neutrophils (PMN), granulocytes with visible segmented nucleus at ×400 magnification in 5 random fields is depicted (t test). k Spleen weight of control and histamine-treated 4T1 tumour-bearing mice (Mann–Whitney test, *P < 0.05 vs. control). l Spearman correlation of spleen weight vs. tumour weight for control mice (r: 0.6508, **P = 0.0019; n = 20) and mice treated with histamine (r: 0.7860, **P < 0.0001; n = 20)