Checkpoint inhibitors, anti-PD1/anti-PD-L1 activities are channeled via CD28 co-stimulation

Two new back-to-back studies in Science showed that T cell recovery activity attributed to checkpoint inhibitors, anti-PD1/anti-PD-L1, is mediated via CD28 co-stimulation. One of the study focused on analysis of biochemical events leading to PD1 signaling in an in vitro reconstitution model and another study provided corroborating data using mouse model. 

Summary results from both studies suggest that anti-PD1/PD-L1 activity is lost when T cells lack co-stimulatory molecule CD28. However, readouts here are more complicated since PD-L1 itself could bind to CD28 ligand B7 molecule. Also, I noticed that by default T cells with inducible CD28 deficiency (CD28^f/f Cre^ERT2+) show less accumulation and/or survival (low cell density) compared to WT counterparts.

Another confusion has to do with the fact that at least in one of the clinical trials anti-CD28 antibodies produced severe cytokine release syndrome and program was discontinued. It could be that anti-PD1/PDL1 therapy activates only certain type of T cells while anti-CD28 could have targeted much larger T cell population.

David Usharauli

  

Tetra-punch against solid tumors

Checkpoint blockade therapies has become a gold standard for cancer immunotherapy. However, only in minority of cancer patients did these antibody therapies show significant benefits. Many think that a multi-pronged approach to cancer therapy could tip the balance in favor of anti-tumor therapy.

For sure, data from mouse studies support this line of thinking. For example, this week Nature Medicine published a mouse study showing dramatic benefits of immunotherapy when four different approaches were combined: 

1. Anti-cancer Antibody (A)

2. Long-lived IL-2 (I)

3. Checkpoint PD1 inhibitor (P)

4. Cancer Vaccine (V)

Referred as AIPV this experimental tetra-pronged immunotherapy could clear an established solid tumors (melanoma, breast cancer, adenocarcinoma) in 75%-80% of mice.

Success of AIPV therapy depended mostly on CD8 T cell and NK cells.

Of note, however, frequency of IFN-γ⁺ CD8 T cells did not correlate with anti-tumor effectiveness.

Interestingly, through AIPV could induced endogenous anti-cancer antibodies that transferred protection in naive hosts against intravenous tumor inoculum, B cell deficient mice were still protected against tumors when immunized with AIPV.

Finally, AIPV protected against autochthonous [endogenously developed] tumor in Braf^CA Pten^loxPTyr::Cre^ERT2 mice.

In summary, this mouse study shows that multi pronged immunotherapeutic approach could significantly improve survival rate during cancer therapy. The authors claimed that "AIPV therapy was associated with minimal systemic toxicity, as mice did not show weight loss or substantial elevation in the amounts of liver enzymes in the blood".

Of course, it is difficult to compare outcome in mouse study versus human study. In humans, even single approach with anti-PD1 antibody frequently leads to lung or liver toxicity. Now imagine injecting cancer patients with 4 different immunotherapeutics. So, we have a long way to go before immuntherapy will show the same acceptable-level effectiveness in humans as it does in lab mice.  

David Usharauli 

   

Cish is a NK cell checkpoint inhibitor

This week several science news outlets spotlighted new study from Nature Immunology showing anti-tumor effect of Cish deficient NK cells. This study showed that Cish deficient NK cells are hyper-responsive to its canonical cytokine IL-15 and show improved control of experimental tumors.

I analysed this study to determine if the buzz was deserving. In my view this study is in fact two independent [and not connected] studies put together artificially. The finding that Cish deficient NK cells have superior anti-tumor behavior is based on non-physiological experimental model.

First part of this study deals with cytokine sensitivity of Cish deficient NK cells. This is exclusively in vitro experiments. It does show that Cish deficient NK cells are hyper-responsive to NK cell canonical cytokines such as IL-15, or IL-12/IL-18 combo [and other activatory receptors].

Second part of this study, however, is completely dissociated with IL-15 story and jumps directly to tumor protection experiments with WT or Cish deficient NK cells. These experiments showed that Cish deficient mice are resistant to exogenously injected tumor cells and this protection depended on asiolo-GM1+ cells and IFN-γ.

Finally, the authors showed that adoptive transfer of Cish deficient NK cells into NK-deficient hosts (Ncr1^Mcl1Δ/Δ mice) also provided protection against i.v. injected melanoma cell metastasis.

In summary, this study proposed that Cish functions as a checkpoint inhibitor for NK cells.

My view:

(A) It is not clear whether Cish deficient NK cells alone is sufficient for anti-tumor effect [it is independent of CD8 T cells, but CD4 T cells involvement was not tested].

(B) It is not clear what role cytokines such as IL-15 or IL-12/18 play in vivo in Cish deficient mice.  

(C) One of the major differences between NK and T cells is that NK cell effector functions are not regulated in an antigen-specific manner but rather by sensing signaling balance between activatory and inhibitory membrane receptors [functions like a rheostat].

During ontogeny individual NK cell undergoes its own "adjustment" to its environment and can even "tolerate" naturally arising tumor cells. This is why experimental tumor models when tumors are injected exogenously, and appear in the body out of the blue so to speak, do not recapitulate natural interaction with NK cells and easily could produce biased, non-physiological response from first-time encountering NK cells. The more physiological tumor models will be to use spontaneously arising mouse tumor models.

David Usharauli

    

PSGL-1, a ligand for the selectin family of receptors, controls T cell immunopathology

The Selectin receptors, L, E and P, control T cell migration. New study in Immunity showed that Selplg-KO T cells (lacking selectin receptor ligand PSGL-1) display enhanced effector differentiation and greater control of chronic viral infection and tumor, though it comes at the expense of tissue pathology.  

In this paper the authors referred to PSGL-1 as checkpoint inhibitor. However, unlike CTLA4 and PD1 deficient mice, Selplg ^-/- naive mice don't show any abnormality. Interestingly, the authors found that Selplg ^-/- mice efficiently controlled chronic LCMV infection (C13).

However, this enhanced protection against chronic LCMV infection led to severe tissue pathology.

Additionally, and most likely reason why this paper ended up in Immunity, the authors showed that Selplg ^-/- mice could better control tumor (Yumm1.5 melanoma cells).

At present it is not clear whether PSGL-1 signaling works as an independent "checkpoint inhibitor" in vivo or modulates functionality of other canonical checkpoint inhibitors such as PD-1 .

David Usharauli