Antitumor immunity is one of the most critical mechanisms for maintaining health by eliminating tumor cells. Dendritic cells (DCs), as highly potent antigen-presenting cells (APCs), play a central role in initiating immune responses. They capture extracellular antigens, process them into peptides, and promptly present these on major histocompatibility complex (MHC) class II molecules to activate CD4+ T cells [1]. Cytokines such as IL-2, interferon (IFN)-γ, and tumor necrosis factor (TNF)-α, produced by CD4+ T cells, promote the differentiation of cytotoxic T lymphocytes (CTLs) [2]. Additionally, DCs exhibit a distinctive ability known as “cross-presentation,” enabling them to activate antigen-specific CD8+ CTLs, thereby contributing to tumor-protective immunity [3]. In general, CTLs produce IFN-γ and Granzyme B and act as key effector cells in mediating tumor cell apoptosis [4].

Metastasis refers to the process by which cancer cells spread from one organ or tissue to another, typically via the bloodstream or lymphatic system. It is the leading cause of cancer-related morbidity and mortality, accounting for approximately 90% of cancer-related deaths [5]. The chemokine receptors CXCR3 and CCR5 on CTLs are upregulated in response to chemokines secreted by cancer cells, directing CTLs to the tumor site [6]. At the metastasis site, activated CTLs release cytotoxic granules and cytokines that trigger apoptosis in metastatic cancer cells.

Lactic acid bacteria (LAB), which produce lactic acid as the primary metabolic end product of carbohydrate fermentation, are widely utilized in production of fermented foods. LAB are known for their beneficial health effects, including modulation of immune responses. Previous studies have shown that LAB cell wall components activate DCs and macrophages, leading to increased production of interleukin (IL)-12 [7], [8]. Oral administration of LAB has been found to activate T cells and natural killer cells, significantly inhibiting cancer cell proliferation [9], [10].

In our previous study, we reported an increase in the abundance of Lactobacillus species during the fermentation of Brassica rapa L. and identified 46 strains of LAB [11]. Among these, Lactiplantibacillus plantarum K4–9 (K4–9) was selected for its superior ability to induce IFN-γ production compared to other strains. As IFN-γ suppresses tumor growth through the activation of DCs and CD8+ T cells, the K4–9 strain may play a role in enhancing antitumor immunity.

In the present study, we examined the antitumor immune effects of the K4–9 strain by analyzing immune cell profiles in mice orally administered K4–9. Spleen cells were isolated from these mice and assessed for immune cell populations, DC activation, and the expression of molecules involved in antitumor responses. Furthermore, the antimetastatic activity of the K4–9 strain was assessed using a mouse model of metastatic colonization.