Differential Uptake of ¹⁸F-fluorodeoxyglucose by Experimental Tumors Xenografted into Immunocompetent and Immunodeficient Mice and the Effect of Immunomodification

Abstract

Purpose

To study the contribution of immunologic background to the uptake of fluorine-18-fluorodeoxyglucose (¹⁸F-FDG) by the tumor tissues.

Methods

The uptakes of ¹⁸F-FDG to the same experimental tumor model (SCCVII) xenografted into immunocompetent and immunodeficient (athymic) mice were compared. In addition, the immunomodifying effect of steroid on the uptake of ¹⁸F-FDG by these tumors was investigated.

Results

The uptake of ¹⁸F-FDG by the tumors in immunocompetent mice was significantly higher than that in immunodeficient (athymic) mice. Although steroid pretreatment had no effect on the tumor uptake in immunodeficient mice, it significantly decreased the tumor uptake in immunocompetent mice.

Conclusion

The higher tumor uptake of ¹⁸F-FDG observed in immunocompetent mice, modulated by steroid pretreatment, was contributed by the host immune reaction, probably cellular immunity employed by T-lymphocytes. These findings can clinically conclude that the intense accumulation of ¹⁸F-FDG in the metastatic lymph nodes, which contain only a small number of cancer cells, was caused by the enhanced uptake of ¹⁸F-FDG by activated T-lymphocytes due to host immunity against cancer cells present in metastatic lymph nodes.

Introduction

Fluorine -18-fluorodeoxyglucose (¹⁸F-FDG) has been approved by the HFCA for clinical application, and is currently extensively used in oncologic patients to detect the enhanced glycolysis of the malignant tumor tissues using positron emission tomography (PET) technique. The usefulness of ¹⁸F-FDG-PET in the management of cancer patients has been reported in various situations, such as differentiating benign from malignant lesions, preoperative staging including lymph node metastasis, follow -up of postoperative patients, evaluation of therapeutic effect, and so on [1,2–5]. Although ¹⁸F-FDG is a valuable cancerseeking agent, it has also been reported to accumulate in acute and chronic inflammatory lesions, granulomatous diseases, and autoimmune diseases [6–8]. In those cases, ¹⁸F-FDG is expected to be taken up by inflammatory cells such as granulocytes, lymphocytes, macrophages, and so on. Because cancer tissues consist of tumor cells and stromata, it is possible that infiltrating cells in the stromata also take up ¹⁸F-FDG. In some specific situations where the tumor was affected by intense host reaction, such as after radiation therapy, ¹⁸F-FDG uptake by tumor tissues often increases despite the decreased viability of tumor cells [9,10]. The main purpose of this study is to evaluate the contribution of nontumorous factors, such as host immune reaction, on the tumor uptake of ¹⁸F-FDG and to study the modifying effect of immunomodulators.

Materials and Methods

SCCVII Xenograft

SCCVII murine ovarian squamous cell carcinoma cells were grown in MEM medium (Nissui Pharmaceuticals, Tokyo, Japan), supplemented with 12.5% fetal calf serum (GIBCO Laboratories, Grand Island, NY) and 0.03% l-glutamine, in a 5% CO₂ environment. A single cell suspension of 2x10⁵ SCCVII cells was subcutaneously injected into the left thighs of female immunocompetent C3H/He or athymic BALB/c nu/nu mice. The SCCVII tumorbearing mice were used for biodistribution study 11 days after the injection of cells.

Production of FDG and Biodistribution Study

Fluorine-18 was produced by a ²⁰Ne (d,α) ¹⁸F nuclear reaction in a cyclotron CYPRIS-325R (Sumitomo Heavy Industries, Tokyo, Japan), and ¹⁸F-FDG was synthesized by the nucleophilic substitution method with a ¹⁸F-FDG synthesizing instrument F-100 (Sumitomo Heavy Industries) [11]. SCCVII tumor-bearing immunocompetent and athymic mice received either a daily intraperitoneal injection of 50 mg/kg methylprednisolone succinate (Sigma-Aldrich Japan, Tokyo, Japan) from the 7th to the 10th day after tumor inoculation. The other group received no treatment and served as a control. These mice were fasted for 4 to 6 hours, and approximately 1.35x10^-3 MBq (50 µCi) of ¹⁸F-FDG was injected in the tail vein of each mouse. One hour after the injection of ¹⁸F-FDG, a group of mice (n=5) was sacrificed by ether inhalation. Tumors, blood, and various organs were removed and weighed, and their radioactivity was counted. The percentages of injected dose per gram of tissue (%ID/g) normalized to 20-g mouse were determined, from which tumor-to-blood or organ-to-blood ratios were calculated.

All animal experiments were carried out in accordance with the regulations regarding animal care and handling and were approved by the animal care committee in the Kyoto University. Statistical analyses of independent variables were performed and a probability value (P) of <.05 was considered significant.

Results

Table 1 summarizes the biodistribution of ¹⁸F-FDG in immunocompetent and athymic mice bearing SCCVII xenografts in a nonsteroid pretreatment group. The striking difference was the significantly higher tumor uptake observed in immunocompetent mice compared to that in athymic mice (9.96±0.47% vs 3.50±0.40%, P<.01). In addition, the blood level of ¹⁸F-FDG was higher in immunocompetent mice, along with uptakes in the spleen and lungs (P<.01). Tumor-to-blood ratio was also higher in immunocompetent mice (21.83±1.33 vs 11.47±2.07, P<.01), whereas liver-to-blood and bone-to-blood ratios were higher in athymic mice. After steroid pretreatment, the tumor uptake of ¹⁸F-FDG was substantially reduced only in immunocompetent mice (from 9.96±0.47% to 7.68±0.55%), but was still significantly higher than that in athymic mice (7.68±0.55% vs 4.11±0.76%, P<.01) (Table 2). The blood level of ¹⁸F-FDG and various organ uptakes were also higher in immunocompetent mice after steroid pretreatment. However, tumor-to-blood and organ-to-blood ratios did not show any significant difference after steroid pretreatment except for stomach-to-blood ratio.

Table 1.

Biodistribution of ¹⁸F-FDG in Immunocompetent and Athymic Mice Bearing SCCVII Xenografts Without Steroid Pretreatment.

Organ	%ID/g		Organ-to-blood ratio
	Immunocompetent	Athymic	Immunocompetent	Athymic
Blood	0.46±0.04*	0.31±0.03	-	-
Liver	0.89±0.04	0.79±0.12	1.96±0.27†	2.57±0.30
Kidney	1.33±0.16	1.08±0.23	2.90±0.31	3.53±0.90
Intestine	2.63±0.43	2.08±0.18	5.75±0.74	6.79±0.83
Stomach	1.57±0.27	1.17±0.21	3.43±0.45	3.87±0.94
Spleen	2.39±0.30*	1.58±0.13	5.25±0.76	5.16±0.63
Lung	2.18±0.18*	1.66±0.12	4.79±0.59	5.45±0.79
Muscle	2.61±1.17	1.43±0.34	5.72±2.44	4.72±1.37
Bone	1.29±0.25	1.61±0.14	2.84±0.56	5.24±0.68
SCCVII	9.96±0.47*	3.50±0.40	21.83±1.33*	11.47±2.07

%ID/g represents percentages of injected dose per gram of tissue after 1 hour of intravenous injection of ¹⁸F-FDG. Values are expressed in mean±SD of five mice.

^*Represents P<.01 between immunocompetent and athymic mice.

^†Represents P<.05 between immunocompetent and athymic mice

Table 2.

Biodistribution of ¹⁸F-FDG in Immunocompetent and Athymic Mice Bearing SCCVII Xenografts With Steroid Pretreatment.

Organ	%ID/g		Organ-to-blood ratio
	Immunocompetent	Athymic	Immunocompetent	Athymic
Blood	0.44±0.05*	0.27±0.04	-	-
Liver	0.68±0.07*	0.47±0.07	1.55±0.20	1.74±0.05
Kidney	1.70±0.21*	1.14±0.14	3.95±0.89	4.22±0.70
Intestine	3.00±0.89†	1.69±0.17	6.75±1.55	6.22±0.35
Stomach	1.06±0.16	1.31±0.14	2.47±0.67*	4.89±0.96
Spleen	2.43±0.24†	1.78±0.31	5.59±0.86	6.55±0.92
Lung	1.87±0.16	1.65±0.53	4.32±0.65	6.05±1.72
Muscle	2.28±0.48†	1.36±0.42	5.23±1.05	5.23±2.23
Bone	1.23±0.22	1.10±0.40	2.86±0.77	4.19±1.96
SCCVII	7.68±0.55*	4.11±0.76	17.56±1.03	15.11±1.83

%ID/g represents percentages of injected dose per gram of tissue after 1 hour of intravenous injection of ¹⁸F-FDG. Values are expressed in mean±SD of five mice.

^*Represents P<.01 between immunocompetent and athymic mice.

^†Represents P<.05 between immunocompetent and athymic mice.

The effects of steroid pretreatment on the biodistribution of ¹⁸F-FDG and also on organ weight in immunocompetent and athymic mice were compared and the results were summarized in Figure 1. In immunocompetent mice, steroid pretreatment significantly reduced the uptake of ¹⁸F-FDG in the liver and SCCVII tumors (P<.01), and also significantly reduced the tumor-to-blood ratio (P<.01). In contrast, in athymic mice, steroid pretreatment did not affect the tumor uptake of ¹⁸F-FDG. Tumor-to-blood ratio rather increased a little. The weight of the spleen significantly decreased after steroid administration in both immunocompetent and athymic mice (Figure 1c), whereas there was no difference in tumor weight in both mice.

Figure 1.

The effect of steroid pretreatment in immunocompetent (C3H/He) and athymic (Balb/c nu/nu) mice bearing xenografted tumors. (a) The effect on percentage of injected dose per gram (%ID/g); (b) the effect on organ-to-blood ratio; and (c) the effect on organ weight. (■) nu/nu without steroid pretreatment (nu/nu—control); (□) nu/nu with steroid pretreatment (nu/nu—steroid); () C3H without steroid pretreatment (C3H—control); and () C3H with steroid pretreatment (C3H—steroid). Mean±SD of five mice: *P <.01 and **P< .05 between immunocompetent and athymic mice.

Discussion

In the present study, the biodistribution of ¹⁸F-FDG in immunocompetent and athymic mice bearing the same SCCVII xenograft was compared along with the effect of steroid pretreatment. The uptake of ¹⁸F-FDG in SCCVII tumor was significantly higher in immunocompetent than in athymic mice. Both mice were transplanted with the same cancer cells, which similarly formed tumor in both mice, with no apparent difference in histology. Therefore, differences in the metabolic activity of cancer cells are not likely a cause of the difference in ¹⁸F-FDG uptakes.] The difference in immunologic backgrounds (i.e., the presence or absence of host immune reaction) is the major factor in the different ¹⁸F-FDG uptakes in the same SCCVII tumors.

Due to the tumorigenicity of SCCVII cells and the availability of athymic mouse, we could not use the identical strain of immunocompetent and athymic mice. However, normal mice of different strains showed similar pharmacokinetics of ¹⁸F-FDG (data not shown). However, it is possible that the pharmacokinetics of ¹⁸F-FDG in immunocompetent mice is not completely identical to that in athymic mice. Actually, ¹⁸F-FDG tended to be cleared more slowly from the blood and some normal organs in immunocompetent (C3H/He) mice than in athymic (BALB/c nu/nu) mice. Therefore, a higher tumor uptake of ¹⁸F-FDG in immunocompetent mice may be contributed, in part, by this slower clearance of ¹⁸F-FDG from the body. However, in addition to percent uptake in the tumor, tumor-to-blood ratio was also significantly higher in immunocompetent mice and the difference in the immunologic background seemed to be the major contributor.

The previous literatures reported two opposite observations. Kubota et al. [12,13] beautifully demonstrated that a high accumulation of ¹⁸F-FDG was shown in macrophages and granulation tissues in experimental syngeneic tumor xenografted in immunocompetent C3H/He mice, which is similar to our model used in the current study. In contrast, Brown et al. [14] reported that ³H-FDG mostly accumulated in viable cells of rat syngeneic breast cancer rather than in macrophages and granulation tissues. We suggested that differences between two observations might be caused by a variability of the host immune response against implanted tumor cells. We were not able to obtain a successful microautoradiography to identify the responsible cells taking up ¹⁸F-FDG using the same method as we used in our previous study [15] because of poor resolution. However, we found numbers of small necrotic foci and mild but diffuse infiltrations of mononuclear cells on hematoxylin and eosin sections of SCCVII tumors growing in both immunocompetent and athymic mice. Therefore, SCCVII tumors would be clearly recognized by the host immune system in both mice. In vivo tumor growth can be allowed, only when host immunity missed the tumor tissue to some extent. Therefore, in case the host immune system did not recognize or fully respond against tumor cells, host cells would not greatly contribute to ¹⁸F-FDG accumulation in tumor tissue as we found in athymic mice.

Steroids are reported to have powerful anti -inflammatory effects and to induce the apoptosis of lymphocytes [15–21]. Some authors have reported the shrinkage of spleen following the use of steroids due to a large -scale destruction of lymphocytes (B and T), which was also observed in the present study. The present investigation showed that steroid pretreatment significantly reduced the tumor uptake of ¹⁸F-FDG only in immunocompetent mice. Because steroid had no effect on the tumor uptake in athymic mice, it is unlikely that this dose of steroid directly affected the metabolic activity of cancer cells. The effect of steroid on activated lymphocytes (especially T-cells) infiltrating in the cancer stroma, therefore, is a possible mechanism of action. In immunocompetent mice, xenografted tumor is recognized by the host immune system, and there should be activated immune cells in the cancer stromata.

Steroids are reported to induce the elevation of plasma glucose level that might change the tumor uptake of ¹⁸F-FDG [22]. However, we did not detect any significant hyperglycemia in other sets of mice, which were injected with the same dose of steroids as we used in this study (data not shown). Because this elevated amount of plasma glucose was much smaller than reported values, which can alter the tissue uptake of ¹⁸F-FDG [22,23], the effect of hyperglycemia to the tumor uptake of ¹⁸F-FDG would not be substantial in this model.

In addition, in our recent work using concanavalin A, a potent stimulator of T-lymphocytes, we have reported that concanavalin A increased the ¹⁸F-FDG uptake both in vitro and in vivo by splenic cells and infiltrating mononuclear cells in the liver, resulting in alteration of the biodistribution of ¹⁸F-FDG in immunocompetent mice [15].

In clinical situations, we sometimes observed that the ¹⁸F-FDG accumulation in metastatic lymph nodes was extremely high in spite of smaller number of cancer cells contained in the lymph node. The findings in the present study could explain the intense uptake of ¹⁸F-FDG by the metastatic lymph nodes because lymph nodes contain a large number of lymphocytes, which can be activated by the presence of cancer cells. These activated immune cells take up ¹⁸F-FDG and make the lymph node uptake of ¹⁸F-FDG very high, and can increase the detectability of metastatic lymph nodes with the clinical ¹⁸F-FDG-PET scan.

In conclusion, the present study showed that a substantial amount of ¹⁸F-FDG uptake by the tumor tissues was contributed by the immunologic response in an immunocompetent host, which could be modulated by immunomodulators.