Mutation in DNAse1L3 triggers systemic lupus erythematosus (SLE)-like condition in mice

Autoimmune disease, as word implies, is an immune response directed to self. Traditionally we used to think that autoimmune diseases arise as a result of failure of adaptive immune system (T and B cells) to distinguish between self and nonself antigens. However, as with many inherited immunodeficiencies, frequently we see that inherited innate genetic mutations play pivotal part in autoimmune phenotypes as well.  

For example, few days ago journal Cell published study showing that mice deficient for DNAse1L3, an enzyme capable of degrading extracellular chromatin associated DNA, develop systemic lupus erythematosus (SLE)-like condition.

DNAse1L3, a homologue to DNAse1, contains a short, positively charged C-terminal peptide that allows it to uniquely digest DNA chromatin in microparticles released from apoptotic cells.

Mice deficient for DNAse1L3 develop anti-dsDNA Ab response and Ab deposition in the kidney glomeruli [clinical feature of SLE in humans].

Unlike other genetic DNA/RNA housekeeping mutations (Trex1^-/-, DNase-II^-/-, RNase H2B^-/- ) phenotype of DNAse1L3-KO mice was independent of STING activity (but was dependent on MyD88, though was not clear how or why).

In summary, This study revealed that DCs/Mac derived secreted DNAse1L3 is intimately involved in digestion of apoptotic microparticle associated DNA and in prevention of anti-DNA antibody formation (of note, one of the authors is a co-founder and consultant of Resolve Therapeutics, which develops soluble nucleases for therapeutic purposes).

David Usharauli

  

Mutation in human interferon regulatory factor-7 (IRF7) predisposes to severe flu infection

Many times, unexpected reactions to infections (as in severe influenza) or innocuous agents (as in allergy) are based on unsuspected mutations in proteins relevant in immune regulation. 

New study in journal Science described the patient with inherited mutation in interferon regulatory factor-7 (IRF7) who developed severe inflammatory reaction to type A influenza (pandemic H1N1).

The authors showed that the patient inherited one mutated copy of IRF7 from each parent. Functionally, the patient's derived IRF7 lacked the ability to induce IFN-α in a reporter assay (parents were heterozygous and had normal IRF7 functions).

In ex vivo experiments the authors showed that patient's derived plasmacytoid dendritic cells (IFN-producing cells) did not respond to H1N1 infection by up-regulating IFN-α system (though small amount of IFN-β was produced).

Furthermore, the authors showed that the patient's derived fibroblast were highly susceptible in a viral replication assay compared to control fibroblast samples, suggesting that high viral titre in this patient could have produced clinical signs of severe flu.

In summary, this short and simple study provides molecular basis in understanding the disease outcome. In the future, when every newborn will have their DNA sequenced, the parents will be in a better position to carry out necessary prophylaxis and avoid any complications from infections or allergy.

David Usharauli

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