In 2006, a life-threatening 'cytokine storm', not predicted by pre-clinical safety testing, rapidly occurred in all six healthy volunteers during the phase I clinical trial of the CD28 superagonist monoclonal antibody (mAb) TGN1412. To date, no unequivocal explanation for the failure of TGN1412 to stimulate profound cytokine release in vitro or in vivo in species used for pre-clinical safety testing has been established. Here, we have identified a species difference almost certainly responsible for this disparate immunopharmacology. EXPERIMENTAL APPROACHPolychromatic flow cytometry and intracellular cytokine staining were employed to dissect the in vitro immunopharmacology of TGN1412 and other therapeutic mAbs at the cellular level to identify differences between humans and species used for pre-clinical safety testing. KEY RESULTSIn vitro IL-2 and IFN-g release from CD4+ effector memory T-cells were key indicators of a TGN1412-type response. This mechanism of cytokine release differed from that of other therapeutic mAbs, which can cause adverse reactions, because these other mAbs stimulate cytokine release primarily from natural killer cells. In contrast to humans, CD28 is not expressed on the CD4+ effector memory T-cells of all species used for pre-clinical safety testing, so cannot be stimulated by TGN1412. CONCLUSIONS AND IMPLICATIONSIt is likely that activation of CD4+ effector memory T-cells by TGN1412 was responsible for the cytokine storm. Lack of CD28 expression on the CD4+ effector memory T-cells of species used for pre-clinical safety testing of TGN1412 offers an explanation for the failure to predict a 'cytokine storm' in humans.

Human cytomegalovirus (HCMV) UL141 induces protection against natural killer cell-mediated cytolysis by downregulating cell surface expression of CD155 (nectin-like molecule 5; poliovirus receptor), a ligand for the activating receptor DNAM-1 (CD226). However, DNAM-1 is also recognized to bind a second ligand, CD112 (nectin-2). We now show that HCMV targets CD112 for proteasome-mediated degradation by 48 h post-infection, thus removing both activating ligands for DNAM-1 from the cell surface during productive infection. Significantly, cell surface expression of both CD112 and CD155 was restored when UL141 was deleted from the HCMV genome. While gpUL141 alone is sufficient to mediate retention of CD155 in the endoplasmic reticulum, UL141 requires assistance from additional HCMV-encoded functions to suppress expression of CD112.

SummaryImmune evasion genes help human cytomegalovirus (HCMV) establish lifelong persistence. Without immune pressure, laboratory-adapted HCMV strains have undergone genetic alterations. Among these, the deletion of the UL/b’ domain is associated with loss of virulence. In a screen of UL/b’, we identified pUL135 as a protein responsible for the characteristic cytopathic effect of clinical HCMV strains that also protected from natural killer (NK) and T cell attack. pUL135 interacted directly with abl interactor 1 (ABI1) and ABI2 to recruit the WAVE2 regulatory complex to the plasma membrane, remodel the actin cytoskeleton and dramatically reduce the efficiency of immune synapse (IS) formation. An intimate association between F-actin filaments in target cells and the IS was dispelled by pUL135 expression. Thus, F-actin in target cells plays a critical role in synaptogenesis, and this can be exploited by pathogens to protect against cytotoxic immune effector cells. An independent interaction between pUL135 and talin disrupted cell contacts with the extracellular matrix.

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With their immunosuppressive features, human mesenchymal stromal cells (MSCs), sometimes also termed as mesenchymal stem cells, hold great potential as a cell‐based therapy for various immune‐mediated diseases. Indeed, MSCs have already been approved as a treatment for graft versus host disease. However, contradictory data from clinical trials and lack of conclusive proof of efficacy hinder the progress toward wider clinical use of MSCs and highlight the need for more relevant disease models. Humanized mice are increasingly used as models to study immune‐mediated disease, as they simulate human immunobiology more closely than conventional murine models. With further advances in their resemblance to human immunobiology, it is very likely that humanized mice will be used more commonly as models to investigate MSCs with regard to their therapeutic safety and their immunomodulatory effect and its underlying mechanisms. Recent studies that explore the immunosuppressive features of MSCs in humanized mouse models will be discussed in this review. Stem Cells 2019;37:298–305