Potentiating Checkpoint Blockade by Cross-Priming Tumor-Reactive T cells with In Situ Vaccination

PROJECT SUMMARY
Checkpoint blockade therapy of cancer has had tremendous impact, but still only a subset of patients
respond. One possible explanation is that some tumor types do not have a sufficient number of somatic mutations
to produce tumor-associated-antigens (TAA) that can be targeted by the immune system, specifically CD8 T
cells. Recent data from our group and others suggest an alternative explanation: there are sufficient TAA, but
also suboptimal cross-presentation of these antigens by suitably activated dendritic cells.
We have proposed that this central problem can be addressed using a novel in situ vaccine approach which
uses 1) Flt3L to recruit DC, 2) radiotherapy to load Flt3L-mobilized DC with TAA, and 3) Toll-like Receptor agonist
(TLRa) to activate TAA-loaded DC for cross-presentation. We carried out an early phase trial testing this
approach and observed partial and complete systemic tumor regressions at distant (untreated) tumors, improving
months after therapy, and even specific elimination of malignant B cells with sparing of healthy B cells,
suggesting a systemic anti-tumor immune response. However, whether this approach actually addressed this
problem of insufficient TAA cross-presentation, and whether this could potentiate subsequent checkpoint
blockade therapy is unknown.
In this proposal, we will investigate the mechanism of the in situ
blockade therapy in a mouse model we have developed and in banked, unidentified samples from two clinical
trials of in situ vaccine alone or with anti-PD1 antibody therapy. First, we will assess clinical samples from the
nearly completed in situ vaccine trial to assess whether the appropriate subsets of DC were recruited and
whether this results in the induction of TAA-specific CD8 T cell responses. Next, we will use several unique
resources in the mouse model, including a novel GFP-
collaborators, a panel of CRISPR gene-edited GFP-expressing lymphoma cell lines, and a mass cytometry
(CyTOF) panel to perform deep profiling of tumor-specific T cells in each therapeutic setting. Finally, we will
assess samples from our newly developed clinical trial (funded by CRI) combining the in situ vaccine with anti-
PD1 antibody therapy to assess whether cross-presentation of both TAA and two surrogate antigens introduced
alongside the ISV (HBsAg and CRM-197) actually occurs and correlates with clinical benefit.
We are well positioned to perform the proposed studies, having generated a large set of preliminary data
indicating not only therapeutic opportunity but also our ability to perform high level immune monitoring of samples
from our patients treated on these novel and promising clinical studies. The proposed studies are important
because they will deepen our understanding of anti-tumor T cell mechanisms and address an urgent and unmet
clinical need for our patients with advanced stage lymphoma, and potentially in the future, for many cancer types.

Grant Number: 4R37CA246239-06
NIH Institute/Center: NIH
Principal Investigator: Joshua Brody