Large-Scale Characterization and Functional Assessment of Adaptive Immunity in Patients with MGUS and SMM Reveals Significant Defects

Introduction

Patients with Monoclonal Gammopathy of Undetermined Significance (MGUS) or Smoldering Multiple Myeloma (SMM) exhibit signs of immune dysregulation despite their early stage of oncogenesis. The SARS-CoV-2 pandemic and subsequent broad vaccination of the population presented an opportunity to compare the real-life immune response of patients and healthy individuals following exposure to a common antigen. Here, we have used SARS-CoV-2 vaccination as a model to study the adaptive immune response and have thus comprehensively assessed the adaptive immune response of patients with MGUS/SMM to SARS-CoV-2 vaccination compared to healthy donors (HD).

Methods

We performed (i) paired 5’ single-cell RNA and T cell receptor (TCR) sequencing on 224 peripheral blood mononuclear cell (PBMC) samples drawn serially before and after two doses of SARS-CoV-2 vaccination (HD, n=26; MGUS, n=20; SMM, n=48; Multiple Myeloma (MM), n=24), (ii) ELISA for anti-Spike IgG antibodies post-vaccination (post-vx), (HD, n=237; MGUS, n=550; SMM, n=947) and MMR antibodies at baseline (HD, n=20; SMM, n=20), (iii) bulk TCR sequencing pre- and post-vx (HD, n=2; SMM, n=2) (Adaptive Biotechnologies), and (iv) IFN gamma ELISpot assay following in vitro stimulation with Spike peptide (HD, n=20; SMM, n=20). All post-vx samples were drawn after 2 doses of vaccination.

Results

Patients and HD showed comparable anti-Spike antibody titers within 14-50 days post-vx, however, we observed faster waning in patients with MGUS/SMM, who had significantly lower titers 50-100 days post-vx (p=0.014 & p=0.007, respectively), suggesting a potential defect in long-lived plasma cell maintenance. Indeed, patients with SMM showed significantly lower titers of antibodies against Mumps and Rubella, antigens encountered in childhood, compared to age-matched HD (p=0.002 & p=0.03, respectively), showing that this defect may also lead to loss of plasma cell memory. Next, we performed single-cell RNA/TCR-seq to assess the patients’ T cell response to vaccination. Patients with SMM showed a significant increase in TCR repertoire diversity post-vx (paired t-test, p=0.046), suggesting at least partial preservation of T cell responses with recruitment of rare clones. Indeed, using bulk TCR-seq, we observed Spike-specific T cells in patients with MGUS/SMM. Interestingly, however, a significant enrichment in Spike-specific clones by single-cell TCR-seq was only observed in HD post-vx (paired t-test, p=0.01), suggesting patient T cell responses may be suboptimal. Functional assessment of T cell activation following stimulation with the Spike peptide showed significantly lower activation in patients with SMM compared to HD (t-test, p=0.002), confirming the suspected defect. Notably, a larger proportion of Spike-specific clones in patients was of the Treg phenotype compared to HD (empirical p=0.002), which may partly explain the observed defect.

Conclusions

This study demonstrates that despite the absence of symptoms, patients with MGUS/SMM may demonstrate defective humoral and cellular immune responses, including loss of plasma cell memory. Specific interventions, such as changes in the vaccination schedule, may be warranted for this population, and the morbidity associated with these defects should be studied further.