Multiple sclerosis (MS) is a complex autoimmune disorder affecting the central nervous system, characterized by demyelination and nerve function impairment (1). MS predominantly affects young adults, typically beginning between 20 to 40 years of age, and is a significant contributor to neurological disability in this age group (2). The disease presents with diverse clinical manifestations and has a multifactorial etiology involving genetic and environmental factors. The main aspect of MS pathology involves the formation of demyelinating lesions, which are predominantly found in the white and grey matter of the brain, along with the spinal cord (1). The immune system and inflammation play crucial roles in the neurodegenerative process of MS (3). From a clinical perspective, MS can exhibit two primary trajectories: relapsing or progressive (4). The most frequent presentation involves relapsing-remitting MS (RRMS), characterized by discrete episodes of neurological dysfunction, followed by partial, complete, or no recovery. However, it is important to acknowledge the growing body of evidence suggesting the presence of progression independent of relapse activity (3, 4). With time, the frequency of RRMS relapses generally diminishes. However, a gradual decline often emerges, leading to a continuous worsening of symptoms, a phase known as secondary progressive MS (SPMS) (1). Diagnosis is based on clinical manifestations, radiological findings (notably MRI T2 lesions), and laboratory evidence (including cerebrospinal fluid-specific oligoclonal bands). These constituent elements collectively adhere to the guidelines outlined in the 2017 McDonald criteria (5).

In recent years, various therapeutic interventions, including disease-modifying therapies (DMTs), have been developed to manage MS effectively. DMTs, such as anti-cluster of differentiation 20 (anti-CD20) monoclonal antibodies, aim to modulate the immune response and ameliorate the disease course. These treatments have revolutionized MS management by targeting B cells, which play a pivotal role in the pathogenesis of MS (3).

The intriguing relationship between anti-CD20 treatments, including rituximab, ocrelizumab, ofatumumab, ublituximab, and immunoglobulin G (IgG) levels has garnered scientific interest. However, its noteworthy to mention that these treatments can lead to low levels of immunoglobulin, known as hypogammaglobulinemia (6). The impact on IgG levels and the subsequent risk of infections varies among individuals and types of MS. Different anti-CD20 therapies may also have variable impacts on IgG levels. While advancements in anti-CD20 treatments have greatly benefited patients, there are inconsistencies and heterogeneity in the available data regarding the connection between these treatments, IgG levels, and the risk of infections. Currently, there is a lack of standardized data to guide physicians in adjusting treatment dosages based on IgG levels. Addressing these knowledge gaps is crucial for enhancing the quality of life for individuals living with MS. However, results from studies in this area have differed.

This review aims to investigate the relationship between anti-CD20 MS treatments, IgG levels, and the risk of hypogammaglobulinemia and infections. By doing so, it aims to inform discussions on anti-CD20 MS treatment strategies and improve treatment decisions to prevent severe infections in patients.

The study was designed and reported with adherence to the Preferred Reporting Items for Systematic Reviews and Meta-analysis (PRISMA) guidelines (7). We submitted the research protocol for this systematic review to the International Prospective Registry of Systematic Reviews (PROSPERO) database¹ and assigned the PROSPERO ID: CRD42024518239. All working group members agreed on the study protocol before beginning the literature search.

To our knowledge this is the first large population-based study investigating the intricate connection between anti-CD20 MS treatments, changes in IgG levels, and the associated risk of hypogammaglobulinemia and subsequent infections. In our comprehensive analysis, we examined 36 studies aiming to elucidate the link between the immunosuppressive impacts of various anti-CD20 monoclonal antibody drugs used in MS and the occurrence of hypogammaglobulinemia, along with the resulting infections. The overall analysis indicates a significant prevalence of hypogammaglobulinemia (11%) among MS patients take anti-CD20 drugs. Examining subgroups based on drug type indicated that rituximab exhibited the highest prevalence at 18%, followed by ocrelizumab at 11%, non-specified anti-CD20 at 10%, and ofatumumab at 2%. Chronic usage of anti-CD20 therapies, such as rituximab and ocrelizumab, has been associated with a decline in immunoglobulin levels, particularly IgG and IgM. The decrease in IgG levels may persist even after the discontinuation of therapy. Also, anti-CD20 therapies, particularly ofatumumab and ocrelizumab, are effective in treating MS but pose a higher risk of infections.

Various disease-modifying therapies for MS differ in mechanisms, efficacy, and safety profiles, with a focus on targeting the immune response (53). Anti-CD20 therapies such as rituximab, ocrelizumab, ofatumumab, and ublituximab have proven effective and well-tolerated in MS patients through clinical trials, thereby expanding treatment options (54). Specifically, the anti-CD20 monoclonal antibodies ofatumumab and ocrelizumab, approved by the US Food and Drug Administration (FDA) and the European Medicines Agency (EMA), demonstrate efficacy in relapsing multiple sclerosis (RMS) by delaying disease progression, reducing relapses, and limiting new lesion formations on brain scans (55, 56). Even though B-cell depleting (anti-CD20) therapy is effective in treating MS patients (32, 57), it comes with the highest infection risk among MS disease-modifying therapies (58).

As a complication of anti-CD20 therapy, a common occurrence is a decline in immunoglobulin levels. In the phase 3 trials for rituximab and ocrelizumab, there was an observed increase in the percentage of patients with serum IgG and IgM levels below the normal range, however, ofatumumab and ublituximab resulted in a rise in the proportion of patients displaying IgM levels below the lower limit of normal (LLN), while not impacting IgG levels (32, 53–59).

Hypogammaglobulinemia can manifest during prolonged anti-CD20 therapy (60, 61). One hypothesis suggests that despite anti-CD20 treatment not directly affecting IgG and IgM-producing plasma cells, it delays the regeneration of B-cells (54). Reports indicate the time required for B-cell regeneration post-treatment: 24 weeks for ofatumumab, 72 weeks (ranging 27–175) for ocrelizumab, and 70 weeks (ranging 0.1–75) for ublituximab (56). Even after 48 weeks from rituximab discontinuation, B-cell levels were merely 30.7% of baseline values (57). Another explanation involves certain B-cell subsets experiencing impaired reconstitution; for instance, post-rituximab therapy, regenerated B cells mainly comprise naïve B cells with fewer differentiated memory B cells (40, 62). These regenerated naive B cells display reduced ability to become plasma cells, leading to decreased IgG and IgA production while still capable of producing IgM (61). This suggests that the impact of anti-CD20 therapies on immunoglobulin levels might involve complex interactions between different types of B cells, affecting humoral immunity (54, 63, 64).

Our overall analysis shows a statistically significant decrease in IgG levels from pre- to post-drug intake and a significant hypogammaglobulinemia development rate among anti-CD20 drugs users. Ofatumumab showed a significant decrease in IgG levels, while ocrelizumab, and rituximab, did not show statistical significance. Saidha et al. (65) conducted a systematic review of clinical trials and real-world evidence (RWE) studies to investigate alterations in immunoglobulin levels in individuals diagnosed with RMS who underwent treatment with either ocrelizumab or ofatumumab, as well as to understand the correlation between changes in Ig levels and the occurrence of infections. The obtained results emphasized that the most frequently documented outcome was the variation in IgG levels. Among the ocrelizumab trial groups, four trial populations observed a decline in IgG levels over 24 to 336 weeks follow-up period. Conversely, in the five ofatumumab trial groups monitored for 104 to 168 weeks, a temporary drop in IgG levels occurred at week 48, but no sustained decrease was noted thereafter. In both ocrelizumab and ofatumumab trials, IgM displayed a declining trend over time. Additionally, a reduction in mean IgA levels was observed in the ocrelizumab treatment group throughout the 336-week follow-up period.

Our analysis also revealed that the duration of drug intake significantly influenced hypogammaglobulinemia development. However, no statistically significant difference was observed in the subgroup analysis based on different treatment durations, indicating that reduced IgG levels within 1 year did not necessarily lead to increased hypogammaglobulinemia with prolonged treatment. In a recent retrospective analysis of a considerable group of individuals with MS who were administered rituximab or ocrelizumab, 3.7% exhibited a decline in their IgG levels, dropping below 5 g/L on average after an exposure period of approximately 29.7 months (44).

Several studies in the literature have sought to investigate the potential association between low IgG levels and demographic characteristics of patients. In a study by Mears et al. (40) involving 184 patients treated with rituximab and ocrelizumab, 22 patients experienced hypogammaglobulinemia. Those with hypogammaglobulinemia were more likely to be aged ≥50 years and exhibited lower initial IgG levels. In another prospective observational study encompassing all patients with MS after undergoing a median of 5 (1–6) cycles of rituximab (32). The research revealed that age was linked to an increased risk of reduced IgG levels below 6 g/L, while there was no significant association with sex or a history of immunosuppressive treatment.

Evidence from certain clinical trials and observational studies has suggested an relationship between Ig antibody levels and infection rates, as well as infection severity in patients with MS (17, 65). Having an improved understanding of such risks is particularly relevant within the context of B cell-depleting therapeutic strategies because one of the main functions of B cells is antibody production. In a broader sense, people with MS taking B cell-depleting therapies that may interfere with the generation and/or release of Ig antibodies in response to infectious exposures may accordingly have a greater risk for serious infections (65). Our study reports an overall infection rate of 11% among MS patients receiving anti-CD20 drugs with ocrelizumab shows the highest prevalence of pulmonary infections (24%), urinary tract infections (10%), gastrointestinal infections (4%), and skin and mucous membrane infections (6%). This infection rate represents the proportion of MS patients who developed hypogammaglobulinemia out of the total number of patients on anti-CD20 drugs.

For rituximab, multiple retrospective studies have indicated that within the MS patient cohort receiving rituximab, a subset displaying reduced levels of IgG experienced escalated rates of severe infections necessitating hospitalization, prolonged antibiotic therapies, or intravenous antibiotic interventions when compared to counterparts with higher IgG levels (44). Another three observational studies encompassing a range of 59 to 1,000 patients, undergoing up to 3.5 years of rituximab treatment, established an augmented infection risk associated with IgG deficiency in MS patients, whereas IgM deficiency did not exhibit a similar association (17, 32, 33). Notably, one study highlighted those patients manifesting diminished IgG levels encountered elevated rates of severe infections even before their IgG levels declined, attributed to factors such as advanced age, extended disease duration, and diminished CD19 count (expressed in pro-B cells, B cells, and short-lived plasma cells) (63). These findings emphasize the intricate interplay of various factors contributing to infection risk beyond solely IgG levels.

Screening for hypogammaglobulinemia is vital as it helps identify patients at risk of severe infections (66). Therefore, initiation of anti-CD20 therapy demands careful monitoring due to the increased potency of infections. However, the medical literature lacks a universally agreed upon threshold for defining hypogammaglobulinemia in MS studies (67). Various definitions of the lower limit of normal (LLN) for immunoglobulins (IgG and IgM) have been utilized across different clinical investigations. The LLN for these immunoglobulins can differ depending on factors such as patient age and the specific clinical laboratory used (67). The range of studies included in our analysis provides a diverse spectrum of reported values for defining hypogammaglobulinemia in MS. Across the studies, the reported cutoff values vary considerably, with the lowest being 5 mg/dL and the highest reaching 7 mg/dL. Considering this broad range, it is evident that there is substantial variability in the literature regarding the threshold for defining hypogammaglobulinemia. To suggest the most suitable cutoff value, a balance between inclusivity and clinical relevance must be struck. Based on the distribution of reported values, a cutoff between 6 and 7 mg/dL emerges as a pragmatic choice, capturing a significant portion of the reported data while maintaining clinical significance. Therefore, a cutoff value of approximately 6–7 mg/dL may offer a reasonable compromise in defining hypogammaglobulinemia in MS studies.

Finally, anti-CD20 therapies have shown a link to higher chances of severe COVID-19 infection and hospitalization, as indicated by real-world data. While there was speculation regarding hypogammaglobulinemia contributing to this heightened risk, a retrospective study involving 758 patients found no significant association between IgG levels below 700 mg/dL and COVID-19 (68). Additionally, in individuals with MS, factors such as advanced age, Black ethnicity, lack of ambulatory ability, existing health conditions, and the use of glucocorticoids have been linked to an increased likelihood of COVID-19 hospitalization or death (53).

This study has some limitations to be addressed. Notably, challenges arose from incomplete data within certain conference abstracts, prompting a fair quality rating for several assessments. Moreover, the study acknowledges the presence of moderate to high heterogeneity in some analyses, may be attributable to differences in populations, ethnicity, and study designs. Mitigation strategies, such as random-effects models and subgroup analyses, were employed, with leave-one-out analyses conducted to validate the precision of estimates across various subanalysis groups. The intricate nature of confounding factors, particularly those associated with infections, poses complexities to the analysis, acknowledging that the considerable challenge in drawing definitive conclusions due to the high degree of heterogeneity. Additionally, the apparent differing rate of hypogammaglobulinemia may be influenced by patient-specific factors that could have varied between the groups receiving different medications, such as disease severity, duration, and individual immune response variations. Further investigation into these potential confounders is warranted to better elucidate the relationship between anti-CD20 MS treatments, IgG levels, and the risk of hypogammaglobulinemia. Also, the study underscores a limitation concerning the scarcity of studies, notably for specific drugs like ofatumumab. The limited data for certain interventions necessitates caution in drawing definitive conclusions, and future research endeavors addressing these gaps could significantly enhance the depth of understanding in this domain.

In conclusion, the study reveals a noteworthy prevalence rate of hypogammaglobulinemia development among MS patients take anti-CD20 therapies, with rituximab demonstrating the highest prevalence of low IgG concentration when compared to other MS drugs. This study contributes valuable insights into the immunosuppressive effects, infection risks, and implications of anti-CD20 therapies in MS treatment. The identified relationships and patterns offer a foundation for clinicians to consider in their risk-benefit assessments and underscore the importance of ongoing monitoring and research to optimize therapeutic strategies and patient outcomes in the context of MS treatment.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

AE: Writing – review & editing, Writing – original draft, Visualization, Validation, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization. NA: Writing – review & editing, Writing – original draft, Methodology, Investigation, Data curation. MA-k: Writing – original draft, Visualization, Resources. LM: Writing – original draft, Resources, Methodology, Investigation. MG: Writing – original draft, Resources, Project administration, Methodology. AF: Writing – original draft, Investigation. HH: Writing – original draft, Resources, Project administration, Investigation. AA: Writing – original draft, Resources, Methodology, Investigation. OA: Writing – review & editing, Writing – original draft, Visualization, Validation, Supervision, Software, Resources, Project administration, Methodology, Investigation, Funding acquisition, Formal analysis, Data curation, Conceptualization.