Improving CAR-T cell anti-tumor selectivity by targeting protein glycoforms

Development of CAR-T cells selectively targeting solid tumors have been challenging due to shared antigenicity between cancerous and healthy tissues. Even minute level of protein [if] expressed on healthy tissue could produce unacceptable side-effects as seen for example in trials with CAR-T cells targeting her2/neu.  

New study in Immunity suggested to target protein glycoforms (here using anti-Tn MUC1 CAR-T cells) instead to avoid cross-targeting of healthy tissue.

The authors speculate that cancer-specific glycosylation could produce cancer-specific glyco-proteins. They found that Tn glycoform of protein mucin 1 (Tn MUC1) is selectively expressed by human T cell leukemia and by many solid tumors and can be specifically detected by CAR-T cells incorporating variable heavy and light chains derived from 5E5 mAb (the University of Copenhagen has patented the 5E5 antibody and antigen epitope and the University of Chicago has filed a patent on the 5E5 CAR).

The authors first tested 5E5BBz CAR-T cells against human Jurkat leukemia cells in vivo in NSG immunodeficient mice and observed that it could double survival of tumor challenged mice.

More importantly, 5E5BBz CAR-T cells were also effective against solid tumor, pancreatic tumor cell line expressing Tn MUC1 glycoform (while the authors reported no side-effects in mice subjected to 5E5BBz CAR-T cells transfer, it is not clear whether human tissue in vivo would express it cryptically).

In summary, the authors believe that targeting glycosylation variants of protein "specifically" expressed on transformed cells could overcome cross-targeting of healthy tissue by CAR-T cells.

David Usharauli

Glycosylation directs IgG affinity selection during immune response

Immunoglobulin (Ig) Fc region determines Antibody effector functionality. In humans, for example, there are four types of IgG molecules that differ in their Fc domain structure: IgG₁, IgG₂, IgG₃, IgG₄. In addition, glycolysation (sugarization) of Fc fragments by syalic acid and fucose influences their binding to inhibitory or activatory Fc receptors.

In this new paper published in journal Cell, Jeffrey Ravetch's lab suggested a mechanism how glycolysation would optimize IgG affinity. This is a hybrid study involving both human and mouse studies. 

The authors have used influenza antigen HA as a model antigen. First, the authors showed that following flu vaccinination in healthy volunteers, overall abundance of sialylated IgG₁ Fc molecules (sFc) correlates with vaccine efficacy as measured by HAI.    

Next, to understand mechanism for improved HAI titers, the authors treated human B cells with immune complexes (ICs) derived either from sialylated (sIC) or asialylated (aIC) antigen-IgG conjugates. This experiments reveal that sialylated ICs induced up-regulation of inhibitory FcRIIb on antigen-specific B cells in a CD23-dependent manner. This could instruct antigen-specific B cells to undergo more stringent affinity-selection in germinal centers.

Antigen-antibody binding studies confirmed that in vivo immunization with sICs could elicit IgG₁ with higher affinity in a CD23-dependent manner (this is a mouse study).

Finally the authors showed that unlike pure HA antigen immunization, immune sera derived from mice immunized with sICs displayed superior activity against flu virus expressing stalk region (this region is a target for universal anti-flu antibodies).

In summary, these results suggest that efficacy of vaccination protocols is influenced by overall abundance of sialylated IgG₁ molecules. However, it is not clear whether this optimization of IgG₁ response are physiologically happening following primary (naive) or secondary (memory) immune response. Another point is whether this particular strategy with sICs is feasible for human vaccination protocols. 

David Usharauli