The following abstract presents results of a study conducted by a contract laboratory for the National Toxicology Program (NTP). The findings have not been peer reviewed and were not evaluated in accordance with the levels of evidence criteria established by the NTP on March 2009 (see http://ntp.niehs.nih.gov/ntp/htdocs/levels/09-3566%20NTP-ITOX-R1.pdf). The findings and conclusions for this study should not be construed to represent the views of the NTP or the US Government.
Concanavalin A is a plant lectin which binds to lymphocyte surface glycoproteins resulting in cell proliferation and agglutination. Succinylation of concanavalin A results in a conversion from a tetrameric to a dimeric form of the molecule decreasing its crosslinking ability. This should result in a compound that is immunostimulatory but not lymphocyte agglutinating. The cell binding portion of the HIV-1 has a significant glycoprotein content and in vitro studies have demonstrated the ability of SCA to impair the infectious nature of HIV-1 through an unknown mechanism. This study was designed to determine the immune effects of SCA.
SCA was obtained from Dr. R. Morris (Stanford University, Stanford, CA). The compound was dissolved in Tris buffer at 25 mg/ml and this stock solution was stored at -70° C. The stock solution was diluted in saline prior to administration. SCA was administered to C57BL/6 female mice by i.p. injection at 0, 0.48, 2.4, and 12.0 mg/kg, seven days per week, for 30 days. A positive control group was treated i.p. with 25 mg/kg of cyclophosphamide (CTX, Sigma Chemical Corporation, St. Louis, MO, purity ~99.4% by HPLC) on 4 consecutive days prior to sacrifice. Thirty mice were used in each treatment regimen with between 6 and 8 animals in a group.
No unusual observations were made on any of the mice during the 30 day dosing period. No SCA-induced effects were observed on body weights. There appeared to be a slight dose-related increase in spleen and thymus weights although these were not statistically significant. In addition, there were no significant observations on any hematological parameter measured. Humoral immune function, as measured by the PFC response to sheep red blood cells, was enhanced following SCA treatment. However, this effect was not statistically significant due to the high variance in the 12.0 mg/kg dose group. Spleen cell proliferative responses to T cell mitogens were suppressed at the sub-optimal concentrations of mitogen only. There was a dose-related suppression of the mixed lymphocyte (MLR) response (50% of control for the 12.0 mg/kg dose group) although this suppression was not statistically significant. Similarly, the cytotoxic T lymphocyte (CTL) response was suppressed to a similar degree at the 12 mg/kg group, a finding which was significant at the 12:1 and 6:1 effector:target ratios. Similar to T cell responses, NK cell function was suppressed by SCA at the highest dosage. The changes in immune function were observed in the absence of any changes in spleen cell subset ratios.
Under these experimental conditions, this study showed that 30 consecutive days of dosing with SCA results in slight effects on immune function in the absence of any overt toxicity. The majority of immune alterations were observed at the high dose, 12.0 mg/kg/day and cannot be accounted for by altered splenic cell populations. The known stimulatory effects of concanavalin A on T cells, over time, may be rendering the host T cells less responsive to T cell stimuli and could possibly explain many, but not all, of the observations in this study.
Table 1: Summary of Succinyl Concanavalin A Immunotoxicity
|Body Weight (g)||21.4||21.6||21.3||20.9|
|CTL Activity 25:1|
|AFC/10E6 Spleen cells||296||349||408||476||6*|
|MLR (cpm x 10E4)||4.87||4.06||2.47||2.47||0.58*|
|NK Cell Activity (100:1)||8.7||8.9||8.0||5.9||3.6*|
|Lymphoproliferation: cpm x 10E3|
Report Date: August 1991