In the context of T-Cell activation and HTLV-1, studies are very limited. Various proteins play a role in inducing T-cell activation, the most important of which are CSK, GSK3β, MAP3K14, PLCG1, PTPN6, SLP-76, TAK1, which were evaluated for mRNA expression in our study. The results of the study indicated the possible induction of increased expression of these genes by HTLV-1. The results showed that virus-infected cells may have a significant increase in the expression of GSK3β, MAP3K14, PLCG1, SLP-76 and TAK1 genes in comparison to healthy population. In contrast, there is no significant changes in PTPN6 and CSK which are negative regulator of T-cell activation in ACs individuals compared to healthy subjects. In general, it seems that activation and proliferation of T lymphocytes is an important part of HTLV-1 pathogenesis.

Modifications were commonly detected in the Tax-interactome and pathways related to T-cell activation. These alterations encompassed NFκB signaling, trafficking, and immunosurveillance pathways [19]. Therefore, irregular activation by Tax within T-cells could potentially hold a significant role in Tax-mediated T-cell activation and transformation [20]. Moreover, some studies indicated that γc-cytokines, specifically IL-2 and IL-15, are generated by CD4 T cells infected with HTLV-1 through transcriptional activation by Tax. Subsequently, these cytokines activate CD8 T cells, leading them to adopt an inflammatory T cell phenotype [21, 22].

Tax, which is one of the most important protein of the HTLV-1 in the context of its pathogenesis, functions as a viral transcription activator protein and plays major role in modulating the expression of cellular genes, particularly in the proliferation of T-cells. This modulation occurs primarily through the activation of the AP-1 and NF-κB. Cells expressing Tax exhibit the ability to circumvent cell-cycle checkpoints, influencing processes related to DNA damage response (DDR) and apoptosis [23]. Consequently, these cells experience genetic and epigenetic alterations, along with modifications in RNA stability [23]. Hence, the activation of signaling pathways by Tax seems to play a role in the initial phases of transformation. Transformed cells subsequently maintain certain constitutively activated pathways crucial for the proliferation and resistance to apoptosis observed in ATL cells [24, 25]. Moreover, there are additional proteins that some studies suggest play roles in T-cell activation. Recent research emphasizes the significance of accessory proteins including p12I, p27I, p13II, and p30II in viral replication and maintaining expression levels in vivo [26]. These proteins not only impact viral infectivity but also enhance T-lymphocyte activation and have the potential to influence gene transcription and mitochondrial function. Specifically, p12I activates NFAT, a crucial T cell transcription factor, while p30II regulates cellular gene expression (26).

C-terminal Src kinase (CSK), a cytoplasmic tyrosine kinase plays a major role in overseeing cellular signal transduction, especially in activation of T-cells. CSK is recognized for its capability to negatively modulate the activity of Src family kinases (SFKs), which contribute to diverse signaling pathways. During T-cell activation, CSK functions as a regulator by phosphorylating a specific tyrosine residue (Tyr-527 in the case of c-Src) within the C-terminal tail of SFKs. This phosphorylation event induces a conformational change that inhibits SFKs' kinase activity, leading to the suppression of their function. [27,28,29,30,31]. An investigation was conducted on C-terminal Src kinase (CSK) within the context of its involvement in T-cell activation and T-cell receptor (TCR) signaling. The research suggests that the presence of the adaptor protein TNF receptor-associated factor 3 (TRAF3) is essential for effective TCR signaling and the normal functioning of T cells. The study, using TRAF3-deficient mouse and human T cells, observed a significant decrease in the activating phosphorylation of the TCR-associated kinase Lck [32]. According to the findings, TRAF3 plays a role in inhibiting Lck inhibitors, such as C-terminal Src kinase (CSK). TRAF3 forms an association with CSK, facilitating its dissociation from the plasma membrane, and it regulates the TCR/CD28-induced localization of another inhibitor, protein tyrosine phosphatase N22 (PTPN22). Loss of TRAF3 leads to elevated levels of CSK and PTPN22 in fractions of T cell membrane. This underscores the significance of TRAF3 in enhancing T cell activation by overseeing the localization and functions of early TCR signaling inhibitors, including CSK [32]. A separate investigation focused on the inhibitory receptor LAIR-1 and its role in suppressing T-cell activation by regulating C-terminal Src kinase (CSK). Contribution of LAIR-1 is to inhibit T-cell receptor signaling by diminishing the phosphorylation of essential components such as LCK, LYN, ZAP-70, and mitogen-activated protein kinases. The intracellular section of LAIR-1, which includes immunoreceptor tyrosine-based inhibition motifs, binds to CSK [33]. This study posits that CSK plays a pivotal role in the LAIR-1-induced inhibition of human TCR signal transmission, suggesting the activation of LAIR-1 as a potential strategy for managing inflammation and addressing autoimmune conditions [33].

Regarding our study, we have seen upregulation of CSK in the ACs compared to healthy individuals, but this upregulation was not significant. Also, negative correlation between Tax and CSK is seen which may explain its non-significant result compared to other T-cell activation associated genes.

GSK3β, or Glycogen Synthase Kinase 3 Beta, is a serine/threonine protein kinase crucial in cellular processes, including T-cell activation. It modulates TCR signaling and, upon engagement, its phosphorylation regulates downstream events. GSK3β 's activity impacts IL-2 production, a key cytokine in T-cell responses. The kinase also influences T-cell differentiation into subsets like Th1 and Th2, affecting the balance of inflammatory responses [34]. Its interaction with beta-catenin in the Wnt pathway further influences T-cell development. Dysregulation of GSK3B is linked to immune disorders. Overall, GSK3β plays a multifaceted role in regulating T-cell functions, ensuring proper immune function and response, and maintaining immune system balance. A study explores the use of GSK3 inhibition in GBM-specific CAR-T cells, demonstrating that pharmacologic inhibition of GSK3 enhances T cell proliferation, reduces exhaustion, and promotes a CAR-T effector memory phenotype. This approach resulted in complete tumor elimination in GBM-bearing animals, suggesting that GSK3 inhibition could be a valuable strategy to improve CAR-T immunotherapy outcomes in solid tumors like GBM [34]. Also, another study discovered that GSK3 beta regulates PDL1 levels suggests a potential impact of inhibiting GSK3 beta on the immune response against tumors [35, 36]. In the context of HTLV-1 infection, A study reported the use of the GSK3β inhibitor, 9-ING-41, in treating ATLL. GSK3β, which implicated in tumorigenesis, was inhibited by 9-ING-41, leading to a durable response in a patient with refractory ATLL. The treatment resulted in a partial response, sustained reduction of ATLL cells, and increased secretion of immune-related molecules by CD8 + T cells [37]. Also, there are various other proteins which have positive effect on T-cell activation. A research investigated the the effect of PLCG1 mutations, particularly S345F, in ATLL. The S345F mutation increases basal PLC activity, contributing to cell proliferation, aggregation, and chemotaxis. Reverting the mutation reduces PLC activity and associated cellular functions in ATL-derived cells [38]. In addition, in accordance to HTLV-1 infection, PLCG1 encodes phospholipase C γ-1 (PLC-γ1), a crucial component in early T-cell receptor (TCR) signaling. Upon TCR activation, PLC-γ1 undergoes tyrosine phosphorylation, initiating the production of two second messengers: inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG). Subsequently, IP3 mobilizes intracellular calcium, crucial for NFAT activation, while DAG stimulates protein kinase C (PKC), facilitating NF-κB signaling [39]. MAP3K14 is involved in the activation of the NF-κB signaling pathway, which plays a role in T-cell activation and immune responses. It participates in the regulation of gene expression and cytokine production. Some studies reported that MAP3K14 is overexpressed in HTLV-1 infected cells and especially in ATLL cells. Overexpression of MAP3K14 leads to T-cell activation [23, 40].

In our study, we have also seen upregulation of GSK3β and PLCG1 which both contribute to T-cell activation and facilitate it. Both upregulations are reported significant despite the negative correlation between Tax and GSK3β. This may show that upregulation of mentioned genes are associated with T-cell activation in ACs compared to healthy individuals.

SH2 domain-containing leukocyte protein of 76 kDa (SLP-76) holds significant importance in T-cell activation. SLP-76 acts as a crucial adaptor protein pivotal in T-cell receptor (TCR) signaling. Its phosphorylation occurs upon TCR engagement, serving as a scaffold for various signaling molecules. This function aids in the activation of downstream signaling pathways which are important for T-cell activation and immune responses. Therefore, SLP-76 plays a facilitative role in T-cell activation, involving ACK1 in its mechanisms [41, 42]. Following our study, we have seen an increased mRNA expression of SLP-76 which may be associated with facilitation in T-cell activation. Virus may use this mechanism for its increased replication and numbers.

PTPN6 is a negative regulator of T-cell receptor signaling. It dephosphorylates and inhibits various signaling molecules, acting as a key player in maintaining the balance of T-cell activation and tolerance. Loss of PTPN6 is showed to be linked with higher NF-KB activation in T-cell lymphoma [43]. Additionally, research has demonstrated that brief STAT3 phosphorylation and activation play a crucial role in immune cell development, including T cells and B cells, as well as functions such as cell growth, proliferation, migration, and death. However, persistent and overactive STAT3 is associated with inflammation, heightened proliferation and survival of cells, increased angiogenesis, metastasis, and disruption of immunity against cancerous cells. PTPN6 acts as a destabilizer of STAT3, resulting in reduced T-cell activation and proliferation [44].

Following our study, there is no significant upregulation of PTPN6 in ACs. This shows that, PTPN6 which acts as T-cell activation suppressor, has no significant changes compared to other genes associated with the activation of this pathway. Also, negative correlation between PTPN6 and Tax is seen.

MAP3K7 (also known as Transforming growth factor-beta-activated kinase 1 or TAK1) is also involved in the activation of T-cells by its contribution in the TGF-β signaling. It can activate downstream kinases, influencing gene expression and cellular responses. An article discussed cellular drug resistance as a major hurdle in cancer chemotherapy, emphasizing the association of chemoresistance with alterations in key pathways such as the MAPK pathway. It indicates TAK1 as a central player, acting as a hub for converging IL1 and TGF-β signaling. TAK1 regulates the phosphorylation and activation transcription factors, mediating inflammatory and pro-survival responses. An article focused on the probable therapeutic effect of targeting TAK1, especially in tumors exhibiting chemoresistance and Epithelial to Mesenchymal Transition (EMT) [45,46,47]. Efforts to design TAK1 kinase activity inhibitors have been made, showing efficacy in preclinical studies when combined with conventional chemotherapeutic drugs. The article covers TAK1 regulation mechanisms and the development of small molecule inhibitors, emphasizing TAK1 as a potential target for improving tumor responses to anticancer therapies. Furthermore, there are studies indicating that TAK1 augments NF-κB activation induced by TGF-β. This is supported by observations where the knockdown of TAK1 through siRNA resulted in a decrease in TGF-β1-induced phosphorylation of IKK, IκB, and RELA. Additionally, there was a reduction diminished transcription of NF-κB-induced reporter and target genes [45,46,47,48,49]. In the context of our study, an increased expression in TAK1 were seen which may be correlated with T-cell activation following HTLV-1 infection in ACs.

Our study also has some limitations. Upregulation of mentioned mRNAs in T-cell activation needs to be confirmed by western blotting. In addition, this study should be conducted on a higher population scale in order to generalize the results to the entire ACs population. One of the other limitations of this study is the lack of access to patients with HAM/TSP and ATLL to compare whether the expression level in these two groups is as high as ACs or it is much different from this group. However, our study provides valuable insights into mRNA expression level changes within the ACs compared to healthy individuals which results in better insights into HTLV-1 pathogenesis.