Regulatory T Cells
Throughout this thesis, the importance of Treg cells in immunotherapy is a recurrent
theme. It is one of the best examples of how the immune system can be a two-edged sword, and how targeted immunotherapy can lead to disease treatment. The findings presented in Chapter 2 illustrate how Treg cells exert their immunosuppression in the post-transplant setting. The
observation that iTreg cells exhibit similar kinetics to suppress GvHD is encouraging, as these
cells can be expanded to larger numbers ex vivo than is possible with eTreg cells. However, the
vivo remains a worrisome possibility. If these cells are shown to be unstable, co-administration with TGF-β, IL-10, or the mTOR inhibitor rapamycin may be able to prevent their de-
differentiation, as well as increase de novo differentiation of inducible Treg cells [27]. Counter-
intuitively, administration of IL-2 may also preferentially support Treg stability and function in
vivo.
In Chapter 4, we showed that claudin-low tumors, one of the intrinsic subtypes of breast cancer, demonstrates high immune cell infiltration and utilizes Treg recruitment to suppress the
anti-tumor response. The large proportion of infiltrating Treg cells can dominate the immune
response, preventing the activation and proliferation of CD8+ effector cells. Even in the absence of adoptive effector cell transfer, depletion of Treg cells delays tumor growth, presumably due to
release of inhibition of effector T cells.
Unfortunately, in both of our experiments, Treg depletion cannot be maintained
throughout the course of the study. Following two to three rounds of Treg depletion, FoxP3DTR
(DEREG) mice eventually reconstitute the Treg pool with non-DTR-expressing Treg cells [29].
Similarly, multiple treatments of low-dose cyclophosphamide cannot be administered, or
activated and proliferating CD8+ effector T cells will be depleted along with the Treg subset. This
is the same mechanism that has limited the utility of CD25-depleting antibodies or IL-2DT
constructs, as activated effector T cells express the IL-2 receptor along with Treg cells [30].
While FoxP3 is specific for Treg cells, it is an intracellular transcription factor and thus
not targetable by antibody inhibition. Identification of specific extracellular Treg markers will be
necessary for the long-term depletion of Treg cells needed for the treatment of solid tumors. In
therapy for the majority of patients treated. However, lessons learned from the study of GvHD can be applied in the tumor setting.
Current work suggests that high expression levels of NRP1 is a specific marker for
endogenous, thymically-derived murine Treg cells, but not inducible Treg cells, and is up-regulated
on Treg cells from cancer patients [31]. Preliminary data from our laboratory suggest that tumor-
infiltrating Treg cells are eTreg cells, as they express high levels of Helios (unpublished data).
However, recent studies have shown that Helios is not always a specific marker for eTreg cells,
leading to an unanswered question of whether tumor-infiltrating Treg cells are endogenous or
induced [32]. In the event that they are induced, other treatment approaches should target the anti-inflammatory signals that lead to their differentiation and function, such as IL-10 and TGF-
β. Conversely, global depletion of all Treg cells is not without its own side effects. FoxP3DTR
(DEREG) adult mice undergoing Treg depletion exhibit adverse symptoms of autoimmunity,
including hunched posture and fur loss, although it does not reach the levels seen with Treg
depletion in neonatal mice (unpublished data). Therefore, the ideal treatment regimen would only target those Treg cells in the tumor microenvironment.
PD-1 and CTLA-4 inhibitory antibodies are showing great promise in clinical trials for many cancers, especially melanoma [33, 34]. Unfortunately, these treatments have shown no efficacy thus far in breast cancer [33-35]. One hypothesis for the absence of activity in patients with breast cancer is the inclusion of individuals with luminal tumors, which do not generate a significant immune response. Instead, rational use of these drugs would target the smaller proportion of breast cancer patients that exhibit heavily Treg-infiltrated tumors, namely triple
showed that depletion of Treg cells with Cy, combined with anti-PD-1 and anti-CTLA-4 antibody
therapy, significantly delayed tumor growth in a claudin-low breast cancer model.
Effector T Cells
Not surprisingly, the treatment benefit observed following Treg depletion and inhibition
was limited to the minority of mice that endogenously develop a CD8+ T cell response. Mice that failed to develop such a response were incapable of mediating tumor regression following therapy. However, this therapy should not be used in isolation. Instead, it can and should be combined with adoptive cell therapy, where CD8+ effector cells can be activated and expanded ex vivo. Upon return to the patient, these cells can mediate tumor cell killing without the immunosuppressive mechanisms that existed during tumor development.
The future clinical utility of adoptive cell therapy for solid tumors will be dependent on our ability to identify tumor-specific antigens. Preferably, these would be neo-antigens
expressed as a result of tumor development, limiting the number of antigen-reactive lymphocytes that underwent central deletion during development. However, even self-antigens that are
enriched in tumor cells can be targeted. Unfortunately, there is currently no easy way to identify all tumor-associated antigens for a given tumor. Given the advances made in next generation sequencing and bioinformatics, future studies should focus on differences in RNA expression between tumor and normal tissue. The use of RNAseq technology in this setting would help to identify antigens that are upregulated on tumor cells or novel antigens resulting from aberrant RNA processing or translation.