Tolerance in humans is a full-time work: CTLA4 haploinsufficiency

CTLA4 is a powerful inhibitory molecule required to keep activated T cells in check. For example, mice deficient in CTLA-4 develop fatal multi-organ immune inflammation, however mice with CTLA4 haploinsufficiency (where only one copy of the gene is defective) showed no apparent clinical signs.

This paper in journal Science describes an identification of 4 different families carrying one defective copy of CTLA-4 gene and (in contrast to mice model) showing immune related inflammatory disorder in multiple organs.

The authors observed that these individuals have reduced number of T regulatory cells with reduced capacity to inhibit T cells in a in vitro assay.

Next, it was found that T regulatory cells from these individuals expressed half the amount of normal CTLA4 level (though the authors failed to compare T regs' functionality sorted based on CTLA4 expression level: high, medium and low).

Additional in vitro experiments showed that transfection of patient's PBMC with healthy, wild-type copy of CTLA4 can reduce T cell proliferative response. 

Conversely, similar to patient's T cells, knock-down of CTLA4 expression by siRNA increased healthy PBMC proliferation (though in both these assays, measuring proliferation by simple gating on dividing population does not provide the full picture of CTLA4's effect).

The authors also observed that these individuals had reduced number of peripheral B cells. 

Finally, in contrast to T cells, in vitro proliferation assay for B cells showed dramatic reduction of proliferation from patient's B cells, probably due to fact that patient's PBMC lacked majority of memory B cells.

In summary, this paper identify the individuals with natural mutations in one copy of CTLA4 gene and having autoimmune phenotype. This indicates that in humans CTLA4 functions is a dose-dependent manner and requires presence of both two healthy alleles. Similar observation was reported by another research group in Nature Medicine (CTLA-4 haploinsufficiency results in disrupted T and B cell homeostasis). In general, results from these studies reminds data from mouse Foxp3 studies where T regs function were shown to be dependent on the level of Foxp3 expression.  

David Usharauli