Efficacy of combination immunotherapy of IgM MAb B6.1 and amphotericin B against disseminated candidiasis

Abstract

We previously reported that the IgM MAb B6.1, specific for β-1,2-mannotriose on the surface of the cell wall of Candida albicans, prevents mice from disseminated candidiasis. The preventive activity of the antibody is mediated by enhanced phagocytosis that caused killing of the fungus and involvement of the complement system. In the present study, MAb B6.1 was tested if the antibody enhances therapeutic efficacy of amphotericin B (Amp B) in a murine model of disseminated candidiasis due to C. albicans. Determination by the kidneys-cfu (colony forming unit) and survival times was used to assess treatment. Mice treated intraperitoneally with MAb B6.1 at 1 h post-infection with C. albicans (5 × 10⁵ yeast cells/mouse) developed fewer 28% cfu and prolonged survival rates than negative controls, whereas administration of B6.1 to mice at 2 h post-infection was ineffective. Therapeutic effect of Amp B on mice with the disseminated disease was dose-dependent, but dosage of Amp B (0.5 mg/kg body weight of mice) was not effective. A combination of MAb B6.1given at 1 h post-infection plus Amp B (0.5 mg dose) enhanced survival times beyond the effect due to only antibody (P < 0.05). The MST (mean survival times) value resulted from the combination therapy-received mice was as almost the same as MST value from 2 mg dose of Amp B-given animals (P < 0.05). In case mice given a combination of Amp B (0.5 mg dose) plus MAb B6.1 at 2 h post-infection — a condition of developing no therapeutic efficacy, the combination also reduced disease severity. All these data indicate that MAb B6.1 acts in concert with Amp B, the antibody enhances therapeutic efficacy of Amp B, and this combination therapy augments protection which implies a possibility of reducing Amp B dose. The augmentation of the response appeared to be specific because an irrelevant IgM carbohydrate-specific MAb was not protective. In conclusion, these studies show that Amp B combined with MAb B6.1 may be an option of solving the problem of limited antifungal drug choices due to drug-resistant C. albicans

Introduction

Candida albicans, a polymorphic opportunistic pathogen, ranks as the fourth most frequently isolated from blood cultures [1] and nosocomial infections [2], [3]. This fungal pathogen causes a clinical significant problem in immunocompromised patients [1], [2], [3], [4], [5]. To the patients who were treated with antifungal drugs, disseminated candidiasis due to C. albicans can be a serious cause which often leads to death [6]. Besides the disseminated disease, this fungus causes local infections like vulvovaginal candidiasis in which C. albicans is the most common etiological agent and thrush [7], [8]. For the treatment of such candidal infections, amphotericin B (Amp B) and the azoles are mostly applied to patients. In fact, Amp B is considered as the drug of choice against the disseminated disease [9], [10], but, unfortunately, there is a certain limit to use Amp B because the antifungal drug causes a severe renal damage [11], [12]. This is because of the poor permeability of Amp B across the cellular membrane [13], and thus an excess of amount of Amp B is usually administered to patients in clinical situations, which causes the renal damage in them. For this reason, ways of reducing the overdose of Amp B have been sought for many years. One of the few ways is to modify formulation of a drug to aid absorption of the drug inside cells. In the case of Amp B, a liposomal formulation of Amp B has been developed, and yet, still, its absorption efficiency seems not to be remarkable [13], [14]. The other way is to mix different kinds of antifungal drugs to each other. However, the resistance to C. albicans is still problematic. Amp B is often combined with the azoles [15], [16], but data reporting resistance of C. albicans to the azoles are increasing, presumably, due to recently increasing numbers of cancer patients [17], [18], [19]. Such limited antifungal drug choices and emergence of the drug-resistant C. albicans strains indicate a necessity for other therapeutic strategies. Thus, a combination therapy of an antifungal drug plus anti-C. albicans antibody, instead of using other chemical drug, may be an alternative methodology. Obviously, this approach of combination immunotherapy would merit consideration. This combination immunotherapy may be beneficial, especially to immunocompromised patients with cancer or AIDS who suffer from Candida infections, possibly by lessening harsh complex therapy with chemical drugs during the process of treatments.

We previously isolated monoclonal antibody specific for β-1,2-mannotriose [20], which is a component of the phosphomannan complex on the cell surface of C. albicans. This monoclonal antibody (MAb), named as B6.1, is protective against disseminated and vaginal forms of the disease [21], [22], [23]. Preventive activity of MAb B6.1 is associated with enhanced phagocytosis and killing of the fungus [24]. In addition, the complement system is essential for the prevention against disseminated candidiasis [25].

Herein, we show that combination of MAb 6.1 plus Amp B enhances therapeutic efficacy against disseminated candidiasis. The antibody acts in concert with Amp B, and this combination immunotherapeutic effect is synergistic. By the benefit of the synergism, a possible reduction of dosage of Amp B would be expected decrease of the severe side effect of Amp B.