Reproductive characteristics of pregnant mares passively immunized against equine chorionic gonadotropin

Graduation date: 1990Two experiments were conducted to examine the effects of passive\ud immunization of pregnant mares with a monoclonal antibody [MAb, 518B7,\ud antibovine βLH which crossreacts with equine chorionic gonadotropin\ud (eCG)] on subsequent reproductive characteristics and changes in plasma\ud concentrations of hormones. In Exp.1 control mares (C, n=3) were not\ud treated. Another group (PGF, n=2) received prostaglandin F₂α (PGF₂α, 5\ud mg, i.m.) on d 45 and every 12 h until abortion. A third group\ud (PGF/MAb, n=4) received PGF₂α as in group PGF plus MAb (35 mg, i.v.) on\ud d 45 and 49. Because there were no significant differences in plasma\ud concentrations of eCG and progesterone between groups PGF and PGF/MAb,\ud data were combined. Treatment of mares with PGF or PGF and MAb caused a\ud transient plateau or slight decrease in plasma eCG concentrations for 2\ud to 24 h compared with that of controls. However, on d 46, 47, 48 and 51\ud eCG concentrations were lower in treated mares (P < .05). Plasma\ud progesterone in treated mares decreased to < 1 ng/ml within 24 h.\ud Abortion occurred in all treated mares (mean ± SE, 5.8 ± 1.0 d) but only\ud four of six mares exhibited post-treatment estrus (d 2, 4, 4 and 28)\ud with a duration of estrus of 4.8 ± 1.0 d (mean ± SE); two mares\ud exhibited estrus before abortion. The interval from treatment to first\ud ovulation was 8.3 ± 1.0 d. There was a tendency for treated mares\ud (aborted) to have fewer luteal structures (secondary corpora lutea or\ud luteinized follicles) than in pregnant control mares (5.2 vs 8.0\ud structures, respectively; P = .13). Proportion of secondary corpora\ud lutea (CL) (66%) to luteinized follicles (33%) was similar between\ud groups. None of the treated mares that exhibited estrus with ovulation\ud subsequent to abortion conceived at breeding; eCG remained elevated\ud until end of the experiment on d 100.\ud In Exp.2 repeated treatment with MAb (MAb, n=3) on d 35 (25 mg,\ud i.v.), 36 (35 mg), 37 (45 mg) and 38 (55 mg) appeared to delay eCG\ud production in treated vs control (C, n=3) mares during this time.\ud However, mean concentrations of eCG were significantly lower (P < .05)\ud on d 39 only in MAb treated mares (.05 IU/ml) but increased after d 40\ud and were similar to that of controls (13.7 IU/ml). Plasma progesterone\ud was lower (P < .05) in treated mares on d 36 only (4.8 vs 7.8 ng/ml in\ud controls). Thereafter, progesterone increased in mares of both groups\ud and did not differ. Treated mares tended to have a higher proportion of\ud luteinized follicles (70%), which occurred later in gestation\ud (mean = d 71); control mares had more ovulatory follicles\ud (78%, mean = d 58).\ud In both experiments treatment with MAb, at the times and dosages\ud used, had only a short-term, transient effect on eCG, whether mares had\ud aborted (Exp.1) or remained pregnant (Exp.2). The significant reduction\ud in plasma progesterone in pregnant MAb-treated mares (Exp.2) on d 36,\ud when eCG is usually low or undetectable, was unexpected. This may\ud indicate a necessity for continued tropic stimulation of the primary\ud corpus luteum by luteinizing hormone before appearance of eCG