Malignant brain tumors have the capability to evade immune surveillance and impede antitumor immune responses, which may lead to continued growth and increased malignancy. In many malignancies, the signal transducer and activator of transcription 3 (STAT-3) plays an integral role in modulating oncogenesis, inhibiting apoptosis, and suppressing immunity [
1,
2]. STAT-3 has been found to be constitutively activated in 50-90% of all malignant tumors, including 53% of anaplastic astrocytomas and 53% of glioblastomas [
3]. In gliomas, cytokines, such as interleukin (IL)-6 (IL-6) [
4,
5] and epidermal growth factor [
5], can cause subsequent phosphorylation and activation of STAT-3. The phosphorylated STAT-3 (p-STAT-3) then translocates into the nucleus and induces a variety of transcriptional factors that propagate tumorigenesis [
1] and up-regulate tumor-mediated immunosuppressive factors [
2]. These factors include IL-10 [
6,
7] that adversely influences Th1-mediated cytotoxic immune responses at multiple levels and is essential for regulatory T cells (Tregs) function [
8,
9], vascular endothelial growth factor [
10] that inhibits dendritic cell maturation and activation by inhibiting co-stimulatory molecule expression [
11], PGE
2 [
12] that induces the immune suppressive Th17 cell [
13], and TGF-β [
14] that induces Tregs, inhibits T cell proliferation and down-modulates the IL-2 receptor. These STAT-3-regulated tumor secreted factors then activate STAT-3 in diverse immune cells [
15] including macrophages and monocytes [
16‐
18], dendritic cells [
2], T cells [
19], and Tregs [
20]. More specifically, IL-2 has been shown to regulate
FoxP3 expression in human CD4
+CD25
+ Tregs by inducing STAT-3 binding of the first intron of the FoxP3 gene [
20]. Because STAT-3 target genes encode many factors that activate STAT-3 in the immune cells, possibly a feed-forward mechanism for activation of STAT-3 in both the tumor cells and the immune cells within the tumor microenvironment is initiated as proposed by Kortylewski [
21]. The cumulative response of activating the STAT3 pathway in the immune system is anti-inflammatory by a combination of suppressing macrophage activation [
22,
23], reducing the cellular cytotoxicity of natural killer cells and neutrophils, reducing the expression of major histocompatibility complex (MHC) II, CD80, CD86, and IL-12 in dendritic cells rendering them unable to stimulate T cells and generate antitumor immunity [
15] and enhancing Treg activity [
20]. Within the immune cells, γ-IFN has been shown to be down-regulated by p-STAT-3 [
15] and accordingly γ-IFN levels have been shown to be decreased in glioma patient PBMCs [
24]. The ablation of STAT-3 activity in only the immune cells results in marked antitumor effects
in vivo, indicating that STAT-3 expression within the immune cells is what restrains antitumor eradication [
15]. Furthermore, we have shown that p-STAT-3 blockade in immune cells restores immune responses [
25] and inhibits Treg induction [
26]. Overall, p-STAT-3 regulates immune suppression and tumor progression via multiple redundant mechanisms [
18,
22,
23,
27,
28].
Primed CD8
+ cytotoxic T cells have been shown to gain central nervous system (CNS) access [
29,
30], and immune cells are present in tumors and the surrounding brain parenchyma [
30]. These immune cells may then traffic outside the CNS [
31,
32] by following the lymphatic drainage through the brain via the Virchow-Robin spaces to lymphatics beneath the cribriform plate, ultimately reaching the cervical lymph nodes [
33,
34]. Thus, CNS tumor-elaborated substances are capable of reaching the immune system and peripheral blood stream. Therefore, we hypothesized that the percent of peripheral blood mononuclear cells (PBMCs) displaying p-STAT-3 may be increased in malignant glioma patients. The p-STAT-3 levels may be increased in the peripheral blood in two ways: (1) a tumor with p-STAT-3 would subsequently induce p-STAT-3 in tumor-associated immune cells, which would then reenter systemic circulation or (2) p-STAT-3 transcriptional induced tumor-secreted products could induce p-STAT-3 in immune cells in the cervical lymph nodes, which then are detected in the peripheral circulation. Therefore, we measured p-STAT-3 in glioma patients' peripheral blood mononuclear cells (PBMCs) and compared these levels to those of healthy donors. We also tested the hypothesis that the level of p-STAT-3 in a tumor would correlate with the percent of PBMCs displaying p-STAT-3. To evaluate whether the percent of PBMCs displaying p-STAT-3 correlated with immune suppression, we tested for a correlation between the percent of PBMCs displaying p-STAT-3 and the fraction of enhanced Tregs in the systemic circulation [
35] especially since p-STAT-3 binds to the first intron of the FoxP3 gene [
20] and because STAT-3 inhibitors have been shown to inhibit Tregs [
26,
36].