[josephst18]

Theory/ educated guess: these therapies try to antagonize the PD1 receptor, which is found on immune cells (specifically, T cells); in a healthy individual, the binding of PD-1L to PD-1 provides a "don't kill me" signal so that the body's own cells are not destroyed by the immune system.

The problem with this is that cancers frequently develop mutations and overexpress PD-1L which prevents the immune system from properly targeting them for destruction. These anti-PD1/ anti-PD1L try to counteract this overexpression, allowing the immune system's T cells to destroy the cancer.

However, targeting the cancer requires a functional immune system; this is where the gut microbiome comes in. It's a sort of "home gym" for the immune system--having a diverse range of bacteria helps keep the immune system strong; gnotobiotic mice (mice bred and raised in completely germ-free environment without any gut microbes) have significantly weakened immune systems.

So theory is that the gut microbiome acts as a "home gym" where the immune system can "train". Anything that reduces gut microbiome (such as antibiotics) will also reduce this "training" and leave the immune system less able to apply its training to fighting off cancer cells.