Background The roles of natural killer (NK) cells, in the process of immunosurveillance and elimination against malignancies, have been increasingly interesting in the space of immuno-oncology. The utilization of humanized mice, reconstituted with human immune cells, improves the translational relevance of animal models in this field. However, it remains a challenge for the effective reconstitution of human NK cells in mice, due to the lack of human cytokines which are essential for the development of NK. Typically, IL-15 humanization could promote the differentiation, function, and survival of NK in such models. Nevertheless, few studies have been conducted to evaluate the impact of IL-15 humanization on a wider range of human immune cell lineages in a comprehensively immuno-reconstituted system.

Methods To understand this, we conducted a study comparing the reconstitution of human NK alone or in a mixture with the autologous PBMC, in the hIL-15-expressing NOD-scid IL2Rg null mice. NCI-H929 human myeloma was established in each mouse, for the evaluation of anti-tumor efficacy by the immunotherapies targeting NK-mediated cell killing. The frequencies of human immune cells in both the circulation and the tumor microenvironment were evaluated by flow cytometry.

Results As a result, the mixed system of NK/PBMC reconstitution achieved a better reconstitution rate of NK cells compared to the NK-alone system. But the anti-tumor efficacy by anti-CD47 or anti-CD38, in contrast to the higher NK frequency, appeared to be poorer in the NK/PBMC-mixed system. To investigate the underlying mechanism, we analyzed the phenotyping of each immune sub-lineages and found an extremely high frequency of hCD4+ cell in the NK/PBMC mixed system.

Conclusions One possible reason for the inversion between NK frequency and drug efficacy might be the enrichment of hCD4+ sub-lineages including Treg, which were promoted by IL-15, suppressed the NK effector function, and compromised the relevant immuno-therapy. Further studies could be suggested including a deeper look into the phenotyping of TILs, combining with a Treg targeting therapy like anti-CTLA4, and studying the mechanism of interaction between Treg and NK. In summary, given the complexity with the function of various immune components co-existing in the system, an appropriate method of immune reconstitution is critical for the evaluation of cancer immuno-therapies utilizing humanized models.