Here’s some strong evidence for what could be the next wave of immuno-oncology: combining a TLR9 ligand with an OX40 antibody. We’re all going to have to get familiar with that sort of talk, so here’s what’s going on:
As those who have worked in inflammation or infectious disease know well, the TLRs (Toll-like receptors) are key sensors in the innate immune system. (As is often the case, especially for proteins of a certain era, the “Toll” nomenclature comes from the Drosophila fruit-fly model where they were first identified, and the human form was named as the analog of that one. hERG is another famous example). They’re found in large part out on the so-called “sentinel cells” (such as macrophages), and the ten TLR subtypes recognize common pathogen motifs. They’re tuned, in other words, to a list of common default settings. For example, TLR5 recognizes bacterial flagellin, which we don’t have, and other TLRs pick up on bacterial lipopolysaccharides or viral nucleic acid varieties. TLR9 recognizes the “CpG” nucleic acid motif that’s found far more in bacteria than it is in higher organisms.
OX40, for its part, is a sort of second-wind system for activated T cells. It’s not even expressed on the surface of the resting cells, but kicks in after 24 hours or so. Its ligand, OX40L, is similar – the antigen-presenting cells only start expressing after a delay as well. T cells start dying off after a couple of days of activation unless they get juiced back up by OX40 activation (the receptor is also known as CD134). I think of it as a “Keep cranking, this is a real one” signal – it’s an immune checkpoint to keep things from going full blast too often. As important as it is that the immune system activates when it should, it’s just as important that it stand down when it should, too (as witness autoimmune disorders, anaphylactic shock, cytokine storms et very much al.)
What this new paper (from the Levy group at Stanford) is doing is injecting tumors with CpG, for starters. That sets off the local CD4 T cell population, and the authors can then see these cells start to express OX40 on their surfaces. This only happens after the addition of exogenous CpG; it doesn’t take place in the normal tumor environment. If you take tumor-infiltrating T cells out in vitro and expose them to CpG, nothing happens on the OX40 front, and nothing happens even inside the tumor if you deplete macrophages and dendritic cells. So it’s these “sentinel cells” that are picking up the presence of CpG and communicating it to the T cells, apparently through IL-12, interferon-gamma, and TNF-alpha or some combination of those (determined through a whole set of other experiments.
So the next step was to do the CpG injection and follow it up with an OX40 agonist to set off those newly expressed receptors. There’s an antibody that binds to OX40 as an agonist, fortunately, and the effects were dramatic. In implanted tumor models in mice, injecting CpG alone showed strong effects at the site of tumor injection, but only there, not at distal tumors. Injecting the OX40 agonist antibody alone showed some effect at all tumor sites, but not enough to be useful. But the combination of the two showed complete regression of both the injected and noninjected tumors. The local CpG injection site went down first, followed after a few days by the distal tumors, consistent with the time it takes to raise a response in both cases.
Only 3 of the 90 mice treated had a recurrance of the distal tumors, but (importantly) these all responded to another round of CpG and OX40 agonist. Remarkably, this combination was effective across a range of implanted tumor types: lymphoma, breast carcinoma, colon, and melanoma. In case you’re wondering, the team did compare the OX40 antibody to PD-1 and PDL-1 antibodies. The latter two did delay tumor growth in the nontreated tumor sites, but were not curative: OX40 treatment was.
The team then went on to an even tougher test, a rodent line that spontaneously develops metastatic tumors in the mammary fat pads. That’s the MMTV-PyMT model, which has been widely used in the field for over ten years now. Injecting the first tumors with CpG and following with the OX40 antibody not only led to regression at the site of injection, it also protected against the development of further tumors. Mice so treated had 70% survival at 25 weeks, compared with 100% mortality at 15 weeks in the control group, which is how you get a P value of <0.0001. Clearly, the immune response raised was both durable and general, not something that was too specific to the first site of injection.
But the injection site was still crucial – a further experiment showed that when more than one antigenically distinct tumor was implanted, injection at either one cleared all the others of its type, but not the other variety. But simultaneous treatment cleared them both.
The authors boldly start off their Discussion section by saying “We have developed a practical strategy for immunotherapy of cancer.” And they may have just done it. I will defer to those with more immunology experience, but personally, I found this paper extremely damned impressive. This treatment mode has a lot of advantages. You don’t need to understand the tumor antigens themselves; you just need a suitable injection site (which will be a limitation for some tumor types, to be sure). The doses required for the injections seem to be quite small, which is also good. The only thing I worry about (always a worry with immune therapies) is what the long-term consequences might be, but when the short term consequences are that all the metastatic tumors disappear that’s not such a bad thing concern to have.
Most interestingly, the exact CpG formulation used in this study is already in human trials. Agonistic OX40 antibodies are also being studied in human trials. This paper immediately and strongly suggests that these two modes of treatment be combined. Cross the streams!