A total of 1,157 records were retrieved from PubMed (n = 274), Embase (n = 606), and Web of Science (n = 277). After de-duplication and initial title and abstract screening, 80 full-text articles were assessed for eligibility based on specific inclusion and exclusion criteria. A total of 19 articles were included in the final review [6, 8, 12, 20, 21, 28,29,30,31,32,33,34,35,36,37,38,39,40,41]. This full process is outlined in Fig. 1.

PRISMA 2020 flow diagram of systematic search investigating the mechanisms underlying SAID development following Alemtuzumab in pwMS

Study and participant information, as well as summarised findings for each of the studies are reported in additional file 2. Out of the 19 included studies: 6 were case–control studies [20, 21, 28, 32, 34, 38], 11 were cohort studies [6, 8, 12, 31, 33, 35,36,37, 39,40,41]. The remaining two studies included an open-label study [30] and a randomised control trial (RCT) [29]. Almost all included studies were conducted longitudinally [6, 8, 12, 28,29,30,31, 33,34,35,36,37,38,39,40,41], except one which was a cross-sectional study [20] and two studies that incorporated a mixed longitudinal and cross-sectional design [21, 32]. Seven of the studies were add-on mechanistic sub-studies for larger clinical trials [6, 8, 21, 32, 35, 36, 40].

Across the included studies, there were a total of 2,236 participants. Due to variability in subgrouping and data format aggregate participant characteristics (e.g. age and sex) means were not calculated. Raw data is presented in Additional file 2. Almost all included participants were diagnosed with relapsing–remitting MS (RRMS) [6, 8, 12, 20, 21, 28, 29, 32,33,34,35,36,37,38,39,40,41], except for two studies that recruited people with progressive MS [30, 31]. All participants were treated with a minimum dose of alemtuzumab of 96 mg except one study, where one participant received a total of 60 mg [31]. Further cycles were administered, as needed, to those who experienced a further relapse. The follow-up duration following alemtuzumab across the studies ranged from 2 to 12 years [6, 8, 12, 20, 28, 29, 31, 33,34,35,36,37, 39,40,41]. Approximately 952.13 (47.92%) pwMS who were treated with alemtuzumab went on to develop SAID. Only 11 studies reported the time of onset of SAID from the first infusion [6, 12, 28,29,30, 33,34,35,36,37, 41]. Time of onset varied between and across the studies, ranging from 6 to 123 months.

Immunological, genetic, endocrine, and neurological markers were measured across the studies, and outcomes have been simplified in Table 1. A variety of different techniques were used across the studies to measure these markers. The most frequently used was flow cytometry which was incorporated in 11 studies either in isolation or in combination with other techniques [6, 8, 21, 29,30,31,32, 38,39,40,41]. Five studies specifically used the specialised flow cytometry technique fluorescence-activated cell sorting (FACS) [6, 8, 32, 38,39,40]. PCR was also used in four studies [21, 30, 32, 33]. In the four studies that measured baseline thyroid autoantibodies, two utilised electrochemiluminescence [34, 35], one utilised ELISA [36], while the remaining study utilised an in-house luciferase assay in combination with flow cytometry [