The central nervous system lesions of multiple sclerosis (MS) can be detected by 
magnetic resonance imaging (MRI) and the initial perivascular inflammatory 
component is distinguished by the presence of gadolinium enhancement. To assess 
the effect of systemic lymphocyte depletion on disease activity, seven patients 
with MS received a 10-day intravenous course of the humanised monoclonal 
antibody CAMPATH-1H (anti-CDw52). With some variations in the protocol, enhanced 
cerebral MR images were obtained monthly for 3-4 months before and at least 6 
months after treatment. 28 enhancing areas were detected on the first series of 
7 scans; 51 additional active lesions were identified on 18 scans before 
treatment; 15 were detected on 20 scans done over the next 3 months, but only 2 
active lesions were seen on 23 scans during follow-up beyond 3 months. The 
difference in lesion incidence rate before and after treatment varied and the 
rate ratio was significantly reduced in only three patients. Collectively, in a 
"meta-analysis", the rate ratios were 0.15 [corrected] (95% CI 0.09-0.24) for 
all seven patients and 0.24 (0.14-0.42; p < 0.001) with exclusion of the patient 
whose scanning schedule differed. The effect of CAMPATH-1H on disease activity 
provides direct, but preliminary, evidence that disease activity in MS depends 
on the availability of circulating lymphocytes and can be prevented by 
lymphocyte depletion. It is too early to say anything about the clinical results 
of treatment with this agent.