Cytokines drive CD4⁺ T cells differentiation into specialized subsets of helper T (Th) cells such as Th1, Th2, Th9, and Th17 cells [1], [2], [3]. Th subsets are functionally distinct and play pivotal roles in coordinating immune responses against pathogens [1]. Each Th subset is characterized by unique cytokine profiles, transcriptional programs, and effector functions. Although these Th subsets are essential for eliminating both intracellular and extracellular pathogens, they can also contribute to inflammation in autoimmune diseases and allergic reactions under specific conditions.

The Th1/Th2 paradigm was established in 1986 with the identification of Th1 and Th2 cells, as these T cells show distinct cytokine requirements for their differentiation and functions [4]. Naïve CD4⁺ T cells differentiate into Th1 cells in the presence of IL-12, which mediate cell-dependent immune responses essential for the clearance of intracellular pathogens, whereas IL-4 drives differentiation toward the Th2 lineage, which supports humoral immunity and host defence against extracellular pathogens, including helminths. [5], [6]. Th1 and Th2 subsets are mutually exclusive, as the development and function of one subset inhibits the other [4].

While Th1 and Th2 cells dominated immunological research for a long time, as T cell immunologists had classified inflammatory and immunologic diseases into either Th1 or Th2-associated diseases. The association of infectious and non-infectious diseases with Th1 and/or Th2 was challenged with the identification of cytokines that are not produced by either Th1 or Th2 cells. This led to the idea of the existence of distinct subsets of Th cells with different effector functions than Th1 and Th2 cells, which began with the discovery of a third subset of Th cells called Th17 cells [2]. The identification of Th17 cells marked a significant advancement in the field.

Unlike Th1 and Th2 cells, which require a single cytokine to initiate their differentiation, Th17 cell development depends on the combined presence of TGF-β and IL-6 [7], [8]. This was the first time it was recognized that two cytokines could counteract each other’s individual functions during differentiation and synergistically give rise to a completely new T cell subset with distinct functions. For example, TGF-β alone promotes the differentiation of Foxp3⁺ regulatory T cells (Tregs), which are immunosuppressive in nature [7]. However, when IL-6 is present alongside TGF-β, it inhibits Foxp3 expression and drives the differentiation of IL-17-producing Th17 cells [7]. This phenomenon offers immunologists a powerful tool to identify novel Th cell subsets by exploring the combinatorial effects of two or more cytokines. In line with this, and similar to Th17 cells, the combination of TGF-β and IL-4 counteracts each other’s individual effects and gives rise to a distinct subset of Th cells that primarily produce IL-9, hence termed Th9 cells [9].

The identification of Th9 cell differentiation began with two seminal papers published in 2008 [9], [10]. These studies demonstrated that naïve CD4⁺ T cells, when cultured with TGF-β and IL-4, differentiate into IL-9-producing cells that do not express signature cytokines of Th1, Th2, or Th17 cells, namely IFN-γ, IL-4, and IL-17, respectively [10]. Prior to this discovery, IL-9 was considered a Th2 cytokine, as its function had primarily been characterized in Th2-biased experimental models such as Leishmania major and helminth infection models [11]. These experimental models were primarily developed to understand the generation and function of Th2 cells in experimental settings.

Historically, IL-9 was initially identified as a T cell growth factor (TCGF) and is part of the common γ-chain (γc) cytokine family, similar to IL-2 [12]. Early sequencing and cytokine assays revealed a novel cytokine with a molecular weight of 32–39 kDa, initially termed p40 or TCGF, before being formally designated as IL-9 [12]. An independent study further demonstrated that TCGF could support the growth of Th2 cells independently of IL-2 [13]. Around the same time, a factor known as mast cell enhancing factor (MEF) was reported to promote mast cell proliferation and function independently of IL-3 [12]. It was later discovered that this factor was, in fact, none other than IL-9 (or TCGF).

Since IL-9 was found to promote mast cell functions, subsequent studies revealed a critical role for IL-9 in allergic inflammation and asthma, as mast cells are one of the key drivers in airway inflammation with histamine produced by mast cells [14], [15]. Genetic association studies revealed that both IL-9 and its receptor are genetically linked to susceptibility to asthma [16]. These studies bring IL-9 to the forefront as a potential therapeutic for allergic asthma and atopy. Moreover, IL-9 was found to be linked to immunity similar to other Th1, Th2 and Th17 cell cytokines. Initial studies indicated that IL-9 promotes the elimination of extracellular pathogens like helminth infection [17]. Given its pleotropic functions in both immunity and non-infectious allergic inflammation, it became critical to identify the cellular source of IL-9, though its production was linked with Th2 cells. However, given the heterogeneity of Th2 cells and effector cytokines production primarily relies on the tissue microenvironment, and under these conditions, IL-9 production by Th2 cells remains elusive. Moreover, post-identification of Th17 cells, which produce more IL-9 than Th2 cells, further supported the idea that IL-9 may not be classified as a Th2 cell cytokine [18]. In addition, non-T cell also contributes to IL-9 production, and mast cell is one of the key producers of IL-9 [19]. The cellular source of IL-9 remained elusive until the identification of Th9 cells. Although IL-9 is produced by several immune cell types, Th9 cells are its predominant source and play a major role in driving the immune pathology associated with allergic inflammation, particularly in asthma. Emerging evidence shows that IL-9 and Th9 cells also contribute significantly to respiratory viral illnesses, including RSV and COVID-19, where they exacerbate airway inflammation and tissue damage. These findings highlight the need for comprehensive characterization of Th9 biology in viral infections to better understand their pathogenic roles and identify potential therapeutic targets.