Equine Protozoal Myeloencephalitis (EPM) is arguably the most common and costly equine neurologic diseases nationwide. The national seroprevalence is >50%, but only 0.5-1% of all horses develops disease during their lifetimes. Some EPM affected horses have decreased immune response. A cell-mediated immune response has been shown to be protective for development of EPM after infection with Sarcocystis neurona in mouse models. Levamisole has been proposed as an adjunctive therapy for EPM to upregulate the cell-mediated immune response based on positive results in other species, but there are very limited studies in equids. We hypothesized that levamisole will upregulate the equine cell-mediated macrophage (M1) dendritic cell (DC1) CD4 T-helper 1 (Th1) CD8 Tc1 immune response in vitro.

The first aim was to determine optimal conditions and effects of levamisole on cellular proliferation. Equine PBMCs were harvested from ten horses seronegative for S. neurona. The cells were cultured alone, or with one of the mitogens: concanavalin A (ConA) or phorbol 12-myristate 13-acetate and ionomycin (PMA/I), or with a combination of the above mitogens and levamisole at several conditions. Cellular proliferation was assessed using a colorimetric bromodeoxyuridine ELISA assay.

The second aim was to determine the ability of levamisole, under optimized conditions, to upregulate the M1 DC1 CD4Th1 CD8 Tc1 response in vitro based on activation and function. PBMCs from the same 10 horses were cultured with each of the following: no stimulation, conA, and levamisole with and without ConA. To determine proliferation of each specific subset, cells were labeled with a fluorescent dye, CellTrace. Proliferation was determined based on dye dilution using flow cytometry. To determine the effects of levamisole on the specific immune response, cell subsets were labeled with fluorescent antibodies for cell surface markers (CD4, CD8, CD21, CD172a, CD14) and dendritic and macrophage activations markers (MHC Class II, CD86). Induction of T-regs was based on FoxP3 expression. Immune phenotypes were determined based on intracellular cytokine expression (IFNɣ, IL4, IL10).

Study results indicate that levamisole alone did not significantly alter PBMC proliferation compared to the response of unstimulated cells. Cells cultured with either ConA or PMA/I resulted in a statistically significant increase (P<0.05) in proliferation compared to unstimulated cells. Cells cultured with ConA and levamisole at 1µg/mL resulted in a significant decrease (P<0.05) in proliferation compared with cells cultured with ConA alone.

Flow cytometry data failed to elucidate the specific immune phenotype that is affected by levamisole. Subjectively, there appeared to be a trend for inceased IFNɣ production by CD14 and CD172a positive cells (macrophages and dendritic cells) and a decrease in IFNɣ production by CD4 and CD8 positive cells (T-lymphocytes).

These results demonstrate that levamisole downregulates ConA stimulated PBMC proliferation. Based on these in vitro results, further studies to determine the effectiveness of levamisole on modulating the equine immune system in vivo and to more specifically evaluate the immune cell subets affected by levamisole are warranted.