Granzyme B regulates antiviral CD8+ T cell responses.

CTLs and NK cells use the perforin/granzyme cytotoxic pathway to kill virally infected cells and tumors. Human regulatory T cells also express functional granzymes and perforin and can induce autologous target cell death in vitro. Perforin-deficient mice die of excessive immune responses after viral challenges, implicating a potential role for this pathway in immune regulation. To further investigate the role of granzyme B in immune regulation in response to viral infections, we characterized the immune response in wild-type, granzyme B-deficient, and perforin-deficient mice infected with Sendai virus. Interestingly, granzyme B-deficient mice, and to a lesser extent perforin-deficient mice, exhibited a significant increase in the number of Ag-specific CD8(+) T cells in the lungs and draining lymph nodes of virally infected animals. This increase was not the result of failure in viral clearance because viral titers in granzyme B-deficient mice were similar to wild-type mice and significantly less than perforin-deficient mice. Regulatory T cells from WT mice expressed high levels of granzyme B in response to infection, and depletion of regulatory T cells from these mice resulted in an increase in the number of Ag-specific CD8(+) T cells, similar to that observed in granzyme B-deficient mice. Furthermore, granzyme B-deficient regulatory T cells displayed defective suppression of CD8(+) T cell proliferation in vitro. Taken together, these results suggest a role for granzyme B in the regulatory T cell compartment in immune regulation to viral infections.

CD25 deficiency causes an immune dysregulation, polyendocrinopathy, enteropathy, X-linked-like syndrome, and defective IL-10 expression from CD4 lymphocytes.

BACKGROUND: Immune dysregulation, polyendocrinopathy, enteropathy, X-linked (IPEX) results in systemic autoimmunity from birth and can be caused by mutations in the transcription factor forkhead box P3 (FOXP3). OBJECTIVE: To determine if Foxp3 is required for the generation of IL-10-expressing T regulatory cells. METHODS: CD4 lymphocytes were isolated from patients with IPEX-like syndromes and activated with antibodies to CD3 and CD46 to generate IL-10-expressing T regulatory cells. RESULTS: We describe a patient with clinical manifestations of IPEX that had a normal Foxp3 gene, but who had CD25 deficiency due to autosomal recessive mutations in this gene. This patient exhibited defective IL-10 expression from CD4 lymphocytes, whereas a Foxp3-deficient patient expressed normal levels of IL-10. CONCLUSION: These data show that CD25 deficiency results in an IPEX-like syndrome and suggests that although Foxp3 is not required for normal IL-10 expression by human CD4 lymphocytes, CD25 expression is important. CLINICAL IMPLICATIONS: Any patient with features of IPEX but with a normal Foxp3 gene should be screened for mutations in the IL-2 receptor subunit CD25.

Identification of a low affinity mannose 6-phosphate-binding site in domain 5 of the cation-independent mannose 6-phosphate receptor.

The 300-kDa cation-independent mannose 6-phosphate receptor (CI-MPR) and the 46-kDa cation-dependent MPR (CD-MPR) are type I integral membrane glycoproteins that play a critical role in the intracellular delivery of newly synthesized mannose 6-phosphate (Man-6-P)-containing acid hydrolases to the lysosome. The extracytoplasmic region of the CI-MPR contains 15 contiguous domains, and the two high affinity ( approximately 1 nm) Man-6-P-binding sites have been mapped to domains 1-3 and 9, with essential residues localized to domains 3 and 9. Domain 5 of the CI-MPR exhibits significant sequence homology to domains 3 and 9 as well as to the CD-MPR. A structure-based sequence alignment was performed that predicts that domain 5 contains the four conserved key residues (Gln, Arg, Glu, and Tyr) identified as essential for carbohydrate recognition by the CD-MPR and domains 3 and 9 of the CI-MPR, but lacks two cysteine residues predicted to form a disulfide bond within the binding pocket. To determine whether domain 5 harbors a carbohydrate-binding site, a construct that encodes domain 5 alone (Dom5His) was expressed in Pichia pastoris. Microarray analysis using 30 different oligosaccharides demonstrated that Dom5His bound specifically to a Man-6-P-containing oligosaccharide (pentamannosyl 6-phosphate). Frontal affinity chromatography showed that the affinity of Dom5His for Man-6-P was approximately 300-fold lower (K(i) = 5.3 mm) than that observed for domains 1-3 and 9. The interaction affinity for the lysosomal enzyme beta-glucuronidase was also much lower (K(d) = 54 microm) as determined by surface plasmon resonance analysis. Taken together, these results demonstrate that the CI-MPR contains a third Man-6-P recognition site that is located in domain 5 and that exhibits lower affinity than the carbohydrate-binding sites present in domains 1-3 and 9.

Biochemical and functional properties of the full-length cation-dependent mannose 6-phosphate receptor expressed in Pichia pastoris.

A glycosylation-deficient, full-length cation-dependent mannose 6-phosphate receptor (CD-MPR) containing a yeast signal sequence was expressed in Pichia pastoris using the constitutive promoter of the PGAP gene. The membrane-bound receptor was solubilized using detergents and purified by pentamannosyl phosphate-agarose affinity chromatography. Equilibrium binding studies identified a binding affinity of 2 nM for the lysosomal enzyme, beta-glucuronidase. To probe the linkage specificity of the recombinant CD-MPR, inhibition binding studies were conducted using non-phosphorylated oligomannoses which demonstrated that Manalpha1,2Man exhibits a 4-fold higher inhibition than Manalpha1,3Man and Manalpha1,6Man. The receptor was capable of associating into oligomeric forms and enzymatic deglycosylation revealed the presence of high-mannose sugars at the single potential N-glycosylation site. Mass spectrometric analysis revealed that the receptor was palmitoylated at the two potential cysteines in its cytoplasmic domain. In conclusion, the full-length CD-MPR produced in P. pastoris is structurally and functionally suitable for crystallization studies.

High-level expression and characterization of a secreted recombinant cation-dependent mannose 6-phosphate receptor in Pichia pastoris.

Mannose 6-phosphate receptors (MPRs) form essential components of the lysosomal enzyme targeting system by binding newly synthesized acid hydrolases with high (nM) affinity. We report the use of Pichia pastoris as a host to efficiently express the extracytoplasmic ligand-binding domain of the cation-dependent mannose 6-phosphate receptor. A truncated and glycosylation-deficient form of the receptor AF-Asn(81)/Stop(155) was secreted into the culture medium, yielding approximately 28mg/L after purification, which is an improvement of 10-100-fold compared to expression in baculovirus-infected insect cells and mammalian cells, respectively. Enzymatic deglycosylation indicated high-mannose sugars at the single potential glycosylation site of Asn 81. The extent and heterogeneity of N-glycans were revealed by applying matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). In the case of AF-Asn(81)/Stop(155), the majority (75%) of the oligosaccharides contained chain lengths of Man(8-10)GlcNAc(2) while Man(11-12)GlcNAc(2) comprised the remaining (25%) N-linked sugars. A comparative MALDI-TOF spectra of Asn(81)/Stop(155) purified from insect cells indicated that Man(2-3)GlcNAc(2) and GlcNAcMan(2-3)GlcNAc(2) share the oligosaccharide pool. The receptor isolated from yeast was functional with respect to ligand binding and acid-dependent dissociation properties, as determined by pentamannosyl phosphate-agarose affinity chromatography. In addition, the protein was biochemically and functionally similar to Asn(81)/Stop(155) expressed in insect cells concerning its oligomeric state and binding affinity to the lysosomal enzyme, beta-glucuronidase (K(d)=1.4nM). These results demonstrate that P. pastoris is a convenient system for the production of large quantities of functional recombinant MPRs suitable for structure-function studies.