Modulation of cellular immunity by antibodies against calreticulin

Calreticulin Fragment 39-272 Promotes B16 Melanoma Malignancy through Myeloid-Derived Suppressor Cells In Vivo

Calreticulin (CRT), a multifunctional Ca²⁺-binding glycoprotein mainly located in the endoplasmic reticulum, is a tumor-associated antigen that has been shown to play protective roles in angiogenesis suppression and anti-tumor immunity. We previously reported that soluble CRT (sCRT) was functionally similar to heat shock proteins or damage-associated molecular patterns in terms of ability to activate myeloid cells and elicit strong inflammatory cytokine production. In the present study, B16 melanoma cell lines expressing recombinant CRT fragment 39-272 (sCRT/39-272) in secreted form (B16-CRT), or recombinant enhanced green fluorescence protein (rEGFP) (B16-EGFP), were constructed for investigation on the roles of sCRT in tumor development. When s.c. inoculated into C57BL/6 mice, the B16-CRT cells were significantly more aggressive (in terms of solid tumor growth rate) than B16-EGFP controls in a TLR4- and myeloid-derived suppressor cells (MDSC)-dependent manner. The B16-CRT-bearing mice showed increased Gr1⁺ MDSC infiltration in tumor tissues, accelerated proliferation of CD11b⁺Ly6G⁺Ly6C^low (G-MDSC) precursors in bone marrow, and higher percentages of G-MDSCs in spleen and blood, which was mirrored by decreased percentage of dendritic cells (DC) in periphery. In in vitro studies, recombinant sCRT/39-272 was able to promote migration and survival of tumor-derived MDSCs via interaction with TLR4, inhibit MDSC differentiation into DC, and also elicit expression of inflammatory proteins S100A8 and S100A9 which are essential for functional maturation and chemotactic migration of MDSCs. Our data provide solid evidence for CRT as a double-edged sword in tumor development.

Immunological activity difference between native calreticulin monomers and oligomers

We have recently demonstrated that the greatly increased immunological activities of recombinant murine calreticulin (rCRT) are largely attributed to its self-oligomerization. Although native CRT (nCRT) can also oligomerize under stress conditions in vitro, whether this phenomenon could occur inside cells and the immunological activity difference between nCRT monomers and oligomers remained unclear. In this study, we illustrated the formation of CRT oligomers in tranfectant cells under “heat & low pH” (42°C/pH 6.5) condition. The mixture of nCRT oligomers and monomers (OnCRT) was obtained after 3 hr treatment of murine monomeric nCRT (MnCRT) under similar condition (42°C/pH 5.0) in vitro. The OnCRT thus obtained was better recognized by 2 monoclonal Abs from mice that had been immunized with oligomeric rCRT. Unlike MnCRT, OnCRT was able to elicit CRT-specific IgG production in mice. OnCRT also stimulated bone-marrow derived dendritic cells (BMDCs) to secrete significantly higher levels of TNF-α, IL-6 and IL-12p40 than did MnCRT in vitro. We postulate that oligomerization of soluble CRT may occur under certain pathophysiological conditions (e.g. ultrahyperpyrexia) and the resultant oligomers may exhibit exaggerated immunostimulating activities, thereby affiliating the inflammatory responses in vivo.

CD1d(hi)CD5⁺ B cells differentiate into antibody-secreting cells under the stimulation with calreticulin fragment

Calreticulin (CRT) is a multifunctional molecule in both intracellular and extracellular environment. We have previously found that a recombinant CRT fragment (rCRT/39-272) could modulate T cell-mediated immunity in mice via activation and expansion of CD1d(hi)CD5⁺ B cells as well as induction of CRT-specific regulatory antibodies. Antibody secreting cells (ASCs) are terminally differentiated B cells responsible for producing antibodies to participate in positive immune response as well as immune regulation. In this study, we demonstrate that rCRT/39-272 differentiates murine CD1d(hi)CD5⁺ B cells into ASCs marked by increased expression of plasma cell-associated transcription factors and production of polyreactive antibodies against DNA and CRT in vitro. Intraperitoneal administration of rCRT/39-272 augmented differentiation of CD1d(hi)CD5⁺ B cells into ASCs in naïve mice or mice with experimental autoimmune encephalomyelitis. Thus, we propose that ASC differentiation and subsequent antibody production of CD1d(hi)CD5⁺ B cells are key steps in CRT-mediated immunoregulation on inflammatory T cell responses.

Adjuvanticity of a recombinant calreticulin fragment in assisting anti-β-glucan IgG responses in T cell-deficient mice

Polysaccharide-encapsulated fungi are the chief source of diseases in immunocompromised hosts such as those infected with human immunodeficiency virus or neutropenia patients. Currently available polysaccharide-protein conjugate vaccines are mainly T cell dependent and are usually ineffective in weakened immune systems. In this study, laminarin, a well-characterized β-1,3-glucan, was conjugated with a prokaryotically expressed recombinant fragment (amino acids [aa] 39 to 272) of calreticulin (rCRT/39-272), which exhibits extraordinarily potent immunogenicity and adjuvanticity in experimental animals. The resultant conjugate reserves the immunostimulatory effect of rCRT/39-272 on naïve murine B cells and is capable of eliciting anti-β-glucan IgG (mostly IgG1) responses in not only BALB/c mice but also athymic nude mice. Laminarin-CRT-induced mouse antibodies (Abs) are able to bind with Candida albicans and inhibit its growth in vitro. In addition, vaccination with laminarin-CRT partially protects mice from lethal C. albicans challenge. These results imply that rCRT/39-272 could be used as an ideal carrier or adjuvant for carbohydrate vaccines aimed at inducing or boosting IgG responses to fungal infections in immunodeficient hosts.