New rat model of Pneumocystis pneumonia induced by anti-CD4(+) T-lymphocyte antibodies

Abstract

The CD4(+) T lymphocyte plays a central role in host defense against Pneumocystis pneumonia but has received only limited attention in rats. CD4(+) T-cell-depleting (OX-38) and nondepleting (W3/25) monoclonal antibodies, which recognize an identical or adjacent epitope, were administered for up to 14 weeks to Lewis rats that had been exposed to PNEUMOCYSTIS: While OX-38 produced a greater decrease in circulating CD4(+) cells than W3/25, both antibody treatments resulted in similar effects on the health of the rats and the levels of Pneumocystis pneumonia, which were milder than those found with corticosteroids. W3/25 also did not enhance the severity of Pneumocystis pneumonia achieved with corticosteroids alone. We conclude that CD4(+) cell function is more important than CD4(+) cell number in host defense against Pneumocystis in the rat and that this new model permits study of opportunistic infections in the rat without the confounding effects of corticosteroids.

FIG. 1.

Sequential changes in mean body weight of Lewis rats during various i.p. treatments. PBS (1.0 ml), methylprednisolone (4.0 mg), 1.0 OX-38 (1.0 mg), W3/25 (1.0 mg), and OX-38 plus W3/25 (1.0 mg) were administered weekly to Lewis rats. Rats were weighed weekly; each data point represents the mean body weight (g) of at least 4 to 14 rats.

FIG. 2.

Pneumcystis organism burden of Lewis rats following anti-CD4⁺ Ab treatments after 8 weeks. PBS (1.0 ml), methylprednisolone (4.0 mg), OX-38 (1.0 mg), W3/25 (1.0 mg), and 1.0 mg of OX-38 plus W3/25 were administered weekly to Lewis rats. After 8 weeks of treatment, rats were sacrificed, and lungs were processed and analyzed for organism burden. Pneumocystis cyst results were obtained from CEV-stained slides prepared from processed lungs; Pneumocystis nucleus (nuc) results were obtained from DQ-stained slides prepared from processed lungs. Each data point represents the respective organism burden of one rat. L.O.D., limit of (microscopic) detection (log₁₀ 5.24 organisms per lung). The horizontal bar represents the mean. ***, P < 0.001 compared to steroid-treated controls.

FIG. 3.

Photomicrographs of representative lung sections from a CS-treated rat and anti-CD4⁺ MAb-treated rat stained with H+E and GMS. A lung section from a CS-treated rat following 8 weeks of treatment is represented in panel A. A lung section from a W3/25 MAb-treated rat following 12 weeks of treatment is represented in panel B. The photos were taken with a Zeiss Axioscope microscope (Carl Zeiss, Inc., Germany) with an attached Spot 2e digital camera (Diagnostic Instruments, Inc., Sterling Heights, Mich.). The larger panels are stained with H+E (magnification, ×200); the inset figures stained with GMS (magnification, ×400) indicate many Pneumocystis cysts in a CS-treated rat (A) and occasional cysts in a W3/25 MAb-treated rat (B). Arrows (insets) indicate cysts; notched arrows (A) indicate macrophages in the vicinity of Pneumocystis cysts.

FIG. 4.

Pneumocystis organism burden of Long-Evans rats following W3/25, methylprednisolone, or combined W3/25- methylprednisolone treatment. Rats received either 1.0 mg of W3/25, 4.0 mg of methylprednisolone acetate (Depo-Medrol), or both. Rats were sacrificed after 7 to 9 weeks of treatment. Each data point represents the respective organism burden of one rat. The horizontal bar represents the mean. ***, P < 0.001 compared to steroid-treated controls.