Here, we report that the antitumor activity elicited by the systemic administration of LC is dependent on NK cells, CD4+
T cells (). Notably, the antitumor activity elicited by LC administration was almost completely abrogated in T cell-deficient and NK cell-depleted mice, in spite of concurrent TAM depletion. Thus, our findings suggest that the antitumor effects of i.v.
LC are unlikely to be mediated by angiogenesis inhibition following TAM depletion, as other studies have suggested.5,7,9,13,15,16,19-21
While we cannot rule out a contribution for LC-induced TAM depletion to the overall induction of T- and NK-cell antitumor activity, we postulate that the main immunosuppressive cells depleted by LC are MDSCs in the blood and spleen. It was interesting to observe that three different cell populations (CD4+
T cells, CD8+
T cells and NK cells) are required for the antitumor effects of systemic LC. This suggests a significant interaction between these cell populations in generating antitumor immunity.
Our findings suggest that TAM depletion following i.v.
LC administration is not mediated by a direct cytotoxic effect, but rather occurs as a result of repeated monocyte depletion that, over time, leads to the reduction of newly emigrating TAM precursor cells. Indeed, we and others have found that the i.v.
administration of LC elicits a marked depletion of monocytes in the blood and bone marrow ().52,53
A recent report also demonstrates that monocytes develop into TAMs under conditions of hypoxia.54
Expanded populations of MDSCs have been described in tumor-bearing mice and humans and these cells appear to play a key role in suppressing adaptive immunity to tumors.26-31,55,56
We found that the treatment of tumor-bearing mice with LC induces significant depletion of CD11b+
MDSCs (). Administration of LC primarily depleted mMDSCs as opposed to nMDSCs. The preferential depletion of mMDSC is important given that mMDSCs have been shown to be more immunosuppressive than neutrophilic MDSCs.57
Augier et al.58
have shown that inflammatory Gr-1+
monocytes not only support tumor growth, but also provide precursors to tolerogenic DCs involved in IL-10 production and regulatory T-cell stimulation.
One drawback to the use of an unspecific cell-depleting agent such as LC is that many different types of phagocytic cells are depleted, including DCs.5
Depletion of DCs by a drug like LC potentially inhibits the development of robust tumor-immunity. However, although there was a significant decrease in DCs in the spleen and TDLNs following LC administration (), we observed that weekly treatments with LC did not lead to overt immune suppression, but instead stimulated robust T cell-mediated antitumor immunity. Thus, the net positive effects of systemic LC administration indicate that the elimination of immunosuppressive MDSCs and TAMs outweighs the potential suppressive effects of DC depletion.
The primary effect of LC was to increase T-cell and NK-cell functionality, as reflected by enhanced cytokine secretion and cytotoxicity responses. The fact that LC treatment increased the production of IL-17 () suggests a novel pathway by which MDSCs may potentially inhibit antitumor immune responses. IL-17 has been demonstrated to play a role in the development of antitumor immunity by reducing large tumor burdens in an IFNγ-dependent manner59
and by other T cell-dependent mechanisms.60
Interestingly, some LC-treated MCA205 tumor-bearing mice underwent complete regression and become resistant to a subsequent tumor challenge with MCA205 tumor cells (data not shown). This observation suggests that restoration of T- and NK-cell functions upon myeloid cell depletion may also lead to the development of robust memory T-cell responses.
In our model, we observed enhanced immune activity against unmodified and relatively non-immunogenic tumor cells. The tumor cells used in this study (MCA205 and CT26 cells) are resistant to in vitro killing by LC (unpublished observations) and thus the antitumor effects observed following LC administration appear to reflect the activation of an antitumor immune response. Thus, we propose that the treatment with an unspecific myeloid cell-depleting agent could be effective if administered together with chemotherapy or radiation therapy, as MDSC depletion may enhance antigen presentation to T cells and their functional activation.