The link between CD47 and CRT is also controversial
The link between CD47 and CRT is also controversial. SIRP-dependent or SIRP-independent, such as the case of calreticulin. It has not reached a consensus which of the factors predominate the process, but the answer to this question will determine the optimal pharmaceutical and clinical design of CD47 targeting strategies. with rituximab (1,6), anti-EGFR mAbs (6,18), and anti-CD56 mAbs (3); and with trastuzumab (16). Moreover, macrophages that are deficient in Fc receptors can still enable phagocytosis once incubated with intact anti-CD47 mAb (1). A Harmane recent original study (3) observed the structural resemblance between CD47-Hu5F9 binding interface and CD47-SIRP binding interface. This is the first definitive evidence that the mechanism of anti-CD47 mAb Hu5F9 involves the disruption of the CD47 intrinsic functions. Overestimation of CD47-SIRP interaction Animal models are Harmane limited in their ability to mimic the complex environment inside the human body. As a result, we feel the need to address some of the limitations that may have affected the interpretations Harmane Harmane of the above experiments. We raise the concern that CD47-SIRP interaction may have been overestimated either by SIRP of high affinity to CD47 in certain mouse models, or by the enhanced CD47 clustering in response to particular anti-CD47 mAb. Thus, these results should be interpreted in reference Harmane to species matched animal experiments and systems. Superior affinity of Sirp in NOD mice A desirable animal model for the investigation of anti-CD47 should faithfully mimic the interaction between human CD47 and human SIRP, elicit robust responses from both the innate and adaptive immunity, and recapitulate side effects of CD47 blockade on RBCs and platelets. The immunocompromised NSG mice have been widely adopted for xenograft experiments. However, the rationale and potential pitfalls of using NSG mice in CD47 research have not been well discussed; and therefore, whether it is the best choice for the study deserves further consideration. Concerns have been raised over the suitableness of the SirpNOD allele in NSG animals to the CD47 study (14,19). The binding affinity between human CD47 and SirpNOD is about 10 times greater than with human SIRP (20). Therefore, CD47-SIRP inhibition could carry more weight in the process of macrophage phagocytosis in NOD-based xenograft models, and thus serves as the dominant factor in relative to ADCC and SIRP-independent functions of CD47. It is conceivable that the effects of disrupting CD47-Sirp would be greatly enhanced and more visible in these animals. In this sense, the observed efficacy in the absence of Fc region may not necessarily indicate Fc-independent mechanisms. Instead, it could be merely due to disrupting the stronger CD47-Sirp interaction that is amplified in the NOD background. This speculation is supported by several non-NOD animal studies (see below). When translating to clinical applications, since the human SIRP does not interact with CD47 as tightly (20), the anti-tumor effect of CD47-SIRP inhibition might not be as significant in human patients. As a result, the real-world efficacy of anti-CD47 therapy could potentially be weaker than expected from the NOD-based xenograft models. The above concern is rather plausible especially in the presence of ADCC. In contrast, in the absence of ADCC, contribution from CD47-SIRP seems limited in the inhibition of phagocytosis. CV1, a SIRP variant with high affinity for CD47, competes with endogenous SIRP for CD47 binding. However, CV1 alone, as an Fc-free antagonist, cannot inhibit cancer growth (15). TTI-621, a recombinant fusion protein composed of the human SIRP N-terminus linked to the Fc of IgG1, loses its efficacy when the Fc fragment is replaced with an IgG4 mutant without ADCC capability, despite maintaining its ability to disrupt CD47-SirpNOD interactions (8). These results indicate that the CD47-SirpNOD interaction does not predominate in the anti-phagocytic process in the absence of ADCC, and that anti-CD47 therapy is only meaningful in combination with ADCC. Nevertheless, it is possible that the CD47-SIRP interaction may have differential response to pro-phagocytic signals of different strengths. The synergy between CV1 and other therapeutic mAbs (15) is so dramatic that possibility for overestimation of CD47-SirpNOD interaction remains in the presence of ADCC. We suggest the researchers consider other animal models that better recapitulate CD47-SIRP interaction in humans whenever available. In addition, immunocompromised hosts lack the immune network that effectively responds to Fc fragments. With a weakened adaptive immunity, complement system, and/or NK cells, the ADCC-dependent efficacy and toxicity can be underestimated in immunocompromised xenograft animals. Syngeneic non-NOD models The concern over the PSEN1 enhanced affinity of SirpNOD encouraged the use of non-NOD syngeneic models, in which mouse tumors are established.