Pre-clinical animal model of CD47-blocking immunotherapy in small-cell lung cancer

This week Journal of Clinical Investigation (JCI) published a new study examining CD47-blocking immunotherapy in small-cell lung cancer. This is a joint study by people affiliated with Forty Seven Inc. and Alexo Therapeutics Inc., two biotech companies which focus on clinical application of anti-CD47 targeting immunotherapy.

Just to remind the readers, that simply put, CD47 is an inhibitory molecule expressed on multiple cell types, including tumors, that act as a "do not eat me" signaling. It is believed that by removing this negative signaling, macrophages could phagocytose tumor cells efficiently and thus restrict tumor growth.

In this study, the authors analyzed effectiveness of blocking of CD47/SIRPα pathway in NSG mouse small-cell lung cancer (SCLC) model. First, the authors showed that human small cell lung cancer cell lines or patient-derived SCLC cells express CD47 and when co-incubated with macrophage and anti-CD47 antibody, such tumor cells are efficiently phagocytosed (though there is no correlation between level of CD47 expression and phagocytosing activity).

Next, the authors showed that treatment with anti-human CD47 antibody inhibited growth of human SCLC cells implanted in immunodeficient NSG mice.

Anti-human CD47 antibody was active in PDX model as well (patient-derived xenograft tumor model in NSG mice).

Similar trend, though less potent tumor growth inhibition was observed with CD47KO SCLC cell line derived via Cas9 editing [suggesting that anti-CD47 antibody provides additional signaling beyond CD47 blocking]. 

To answer some of the criticism of earlier studies, the authors conducted CD47KO mouse tumor cell transplantation experiments in immunocompetent mice and observed similar outcome.  

In summary, this study showed that anti-CD47 therapy could be effective against CD47+ small cell lung cancer which originates from neuroendocrine cells of the lung (however keep in mind that in many models, CD47 blockade does not completely eliminate tumors). 

David Usharauli

Monotherapy with anti-CD47, a "do not eat me" signal", may not be sufficient for cancer immunotherapy

This week PNAS published research article that should be interesting for people following advances in cancer immunotherapy. This new study showed that in mice neither monotherapy with anti-CD47 antibody alone nor its combination with cancer-specific antibody could provide any lasting anti-cancer benefit. However, when combined in addition with anti-PDL1 checkpoint inhibitor, triple combination delivered long-lasting cancer protection.

For this study the authors generated novel high affinity anti-mouse CD47 nanobody that could enhance in vitro cancer cell [melanoma] phagocytosis by macrophages when combined with cancer-specific antibody.

However, in vivo, CD47 nanobody, A4, failed as a (1) monotherapy against melanoma challenge, (b) it couldn't enhance anti-cancer effect when combined with cancer-specific antibody (TA99), (c) it couldn't enhance anti-cancer effect of GM-CSF–producing B16F10 cells vaccine (GVAX) and (d) it slightly improved tumor protection when combined with checkpoint inhibitor PD-L1 antibody.

But, when CD47 nanobody, A4, was combined in triple combination with cancer-specific antibody (TA99) and checkpoint inhibitor PD-L1 antibody, it delivered long-lasting tumor protection in 60% of recipients against primary as well as to secondary tumor challenge indicating tumor-specific memory generation.

In summary, this study revealed that anti-CD47 antibody alone showed minimal activity when used against tumor in hosts with intact immune system (senior author of this study is involved in biotech company focusing on CD47 application). The authors suggested that earlier studies reached different results because they used immunodeficient mouse models [NOD-scid, IL2rgKO (NSG) mice] that lack intact adaptive immune system.

David Usharauli

Mismatch between CD47 and SIRPα was found to explain age-old immunogenicity mystery

Sheep erythrocytes (SRBCs) have been used in immunological research for a very long time. It was well documented that SRBCs were highly immunogenic if injected in mice, so researchers were using it to serve as a "carrier" to enhance immune response to other, less immunogenic antigens, epitopes or haptens. Its mechanism of action was not clear.  

Now, new study in journal Immunity has showed that (a) mismatch between sheep CD47 and mouse SIRPα determined SRBCs immunogenicity in vivo, and (b) that absence of CD47 from mouse erythrocytes could convert mouse erythrocytes into "immunogenic".  

First, the authors showed that plate coated mouse SIRPα would only bind wild-type mouse erythrocytes (red blood cells). This confirmed that "do not eat me" inhibitory circuit mediated by CD47-SIRPα interaction are species-specific (though interestingly SIRPα from non-obese diabetic-prone NOD mice strain does bind strongly to human CD47).

Next the authors showed that similar to SRBCs but unlike to wild-type mouse RBCs, erythrocytes from CD47^-/- mice could activate mouse spleen dendritic cells upon adoptive transfer. (A) Interestingly, however, transfer of white blood cells from CD47^-/- mice did not activate spleen DCs; (B) Also, note that the authors failed to observe activation of spleen DCs cultured with SRBCs or CD47-deficient mouse erythrocytes in vitro.

More importantly, additional experiments showed that RBCs from CD47^-/- mice could serve as a "carrier" to enhance CD4 T cell immune response to nominal antigen, such as ovalbumin.

In summary, this study provided long overdue explanation as for immunogenicity of SRBCs. It appears that in absence of CD47-SIRPα inhibitory engagement between SRBCs and mouse spleen DCs, signals [driven by integrins and Src-family tyrosine kinases] dominates that lead to DC activation.

David Usharauli