Cancer immunotherapy has changed the way many cancers are treated. Yet only a proportion of patients achieve strong and durable clinical benefit. For this reason, researchers are looking beyond the first wave of immune checkpoint targets and focusing on signals that help T cells stay active for longer.
 

One of these signals is the OX40/OX40L pathway.
 

OX40, also known as TNFRSF4, is a costimulatory receptor mainly found on T cells after they have been activated by antigen. Its ligand, OX40L, also known as TNFSF4, is mainly expressed on antigen-presenting cells such as dendritic cells. When OX40 and OX40L bind to each other, they provide T cells with an additional activation signal. This signal helps T cells survive, expand, and maintain their immune function.
 

This makes the OX40/OX40L pathway important in two very different disease settings.
 

In cancer, activating OX40 may help strengthen antitumor T-cell responses. This is why OX40 agonist antibodies, such as INCAGN01949, have been developed for solid tumors. In inflammatory and autoimmune diseases, however, the same pathway can contribute to unwanted T-cell activation. In this setting, blocking OX40L may help reduce pathological immune responses. The anti-OX40L antibody amlitelimab has shown clinical efficacy in patients with moderate-to-severe atopic dermatitis, supporting the therapeutic value of this pathway in inflammatory disease [1,2].
 

The biology of this pathway is straightforward but powerful. OX40 is induced after T-cell activation, while OX40L is expressed on dendritic cells and other antigen-presenting cells. Their interaction acts like a “second signal” that helps sustain the immune response after the initial antigen recognition step. Previous studies have shown that OX40L expressed by human dendritic cells can promote the proliferation of naïve CD4⁺ T cells and enhance cytokine production. This indicates that OX40/OX40L is not just a late accessory pathway. It is an important regulator of T-cell expansion and effector immune responses [3].
 

Because of this biology, drug development around OX40/OX40L has moved in two major directions.
 

The first strategy is to activate OX40 signaling. This approach uses OX40 agonist antibodies or OX40L-based fusion proteins to amplify antitumor immunity. The goal is to make T cells more persistent and more effective against tumor cells.

The second strategy is to block the OX40/OX40L interaction. This approach aims to reduce abnormal T-cell activity in diseases such as atopic dermatitis, graft-versus-host disease, and other inflammatory disorders. In simple terms, the same pathway can be pushed in opposite directions depending on the disease context: activation for cancer, blockade for inflammation [1-3].

For academic researchers and pharmaceutical companies, this creates an important experimental challenge. Many OX40- or OX40L-targeting drugs are antibodies. These antibodies usually recognize the extracellular domains of OX40 or OX40L. However, the extracellular regions of human and mouse proteins are not identical. Differences in amino acid sequence, three-dimensional structure, and antibody-binding epitopes can affect how a therapeutic antibody binds and functions in vivo.

This is why conventional mouse models are often not enough for evaluating human OX40/OX40L-targeting drugs.
 

A humanized OX40/OX40L mouse model is designed to address this problem. In this model, mouse Ox40 exons 1–5, which encode the extracellular domain, are replaced with the corresponding human OX40/TNFRSF4 exons. At the same time, mouse Ox40l exons 2–3, which encode the extracellular region, are replaced with human OX40L/TNFSF4 exons.

This design creates a more human-like OX40–OX40L interaction interface inside a living mouse. At the same time, it preserves the broader mouse immune system environment. As a result, the model can be used to evaluate OX40 agonists for cancer immunotherapy, as well as OX40/OX40L blocking agents for atopic dermatitis, graft-versus-host disease, and other inflammatory diseases.
 

In short, the OX40/OX40L humanized mouse model provides a practical bridge between molecular mechanism and preclinical drug evaluation. It allows researchers to study human-relevant receptor–ligand binding, antibody recognition, pathway activation, pathway blockade, and in vivo immune responses in one integrated system. For both academic research and drug development, it offers a more clinically relevant platform for studying one of the key costimulatory pathways in modern immunology.

 

[1] Davis EJ, Martin-Liberal J, Kristeleit R, et al. First-in-human phase I/II, open-label study of the anti-OX40 agonist INCAGN01949 in patients with advanced solid tumors. J Immunother Cancer. 2022;10(10):e004235.

[2] Weidinger S, Bieber T, Cork MJ, et al. Safety and efficacy of amlitelimab, a fully human nondepleting, noncytotoxic anti-OX40 ligand monoclonal antibody, in atopic dermatitis: results of a phase IIa randomized placebo-controlled trial. Br J Dermatol. 2023;189(5):531-539.

[3] Ohshima Y, Tanaka Y, Tozawa H, Takahashi Y, Maliszewski C, Delespesse G. Expression and function of OX40 ligand on human dendritic cells. J Immunol. 1997;159(8):3838-3848.