What are plasma cells, their function, and how are plasma‑cell neoplasms such as multiple myeloma diagnosed and treated?

Plasma Cells: Function, Neoplasms, and Clinical Management

What Are Plasma Cells?

Plasma cells are specialized, terminally differentiated B lymphocytes that synthesize and secrete antibodies to maintain humoral immunity. 1 These cells develop from B lymphocytes through a tightly regulated process in healthy individuals, producing polyclonal, phenotypically normal plasma cells that comprise less than 5% of bone marrow cellularity. 2

Normal Plasma Cell Characteristics

• Phenotypic profile: CD38+/CD138+/CD19+/CD56-/CD45+ with a polyclonal kappa:lambda light chain ratio (0.26-1.65) 2
• Distribution: Scattered individual cells, typically <3-5% of total marrow cellularity 2
• Lifespan: Two distinct populations exist—short-lived plasmablasts that live only as long as B cells are activated, and long-lived non-proliferating memory plasma cells that survive in bone marrow niches for months, years, or a lifetime independent of B or T cell help 1

Plasma Cell Neoplasms: The Disease Spectrum

Multiple myeloma (MM) represents the malignant end of a continuum that invariably begins with monoclonal gammopathy of undetermined significance (MGUS), progressing through overlapping oncogenic events rather than sudden transformation. 2 This is a neoplastic disease characterized by proliferation and accumulation of B-lymphocytes and plasma cells that synthesize monoclonal immunoglobulin (M-protein) in the bone marrow. 3

Disease Classification and Progression Risk

The disease spectrum is defined by three distinct stages with specific diagnostic criteria:

MGUS (Monoclonal Gammopathy of Undetermined Significance):

• Serum monoclonal protein <3 g/dL 3
• Clonal bone marrow plasma cells <10% 3, 2
• Absence of end-organ damage (no CRAB criteria) 3
• Progression risk: 1% per year to myeloma or lymphoproliferative disorders 3, 4

Smoldering Multiple Myeloma (SMM):

• Serum monoclonal protein ≥3 g/dL and/or clonal bone marrow plasma cells ≥10% 3, 2
• Absence of end-organ damage 3
• Progression risk: 10% per year for the first 5 years, 3% per year for the following 5 years, then 1.5% per year thereafter 3, 4

Symptomatic Multiple Myeloma:

• Clonal bone marrow plasma cells ≥10% or biopsy-proven plasmacytoma 3
• Evidence of end-organ damage (CRAB criteria): Calcium elevation (>11.5 mg/dL), Renal insufficiency (creatinine >2 mg/dL), Anemia (hemoglobin <10 g/dL or ≥2 g/dL below normal), Bone lesions (lytic lesions, severe osteopenia, or pathologic fractures) 3, 5

Diagnostic Approach: A Structured Algorithm

Step 1: Initial Laboratory Screening

When bone disease or plasma cell disorder is suspected, immediately order the following comprehensive panel: 5

• Complete blood count with differential 3
• Comprehensive metabolic panel (including serum creatinine and calcium) 3
• Serum protein electrophoresis with immunofixation 3, 5
• Serum free light chain assay with kappa/lambda ratio 3, 5
• Quantitative immunoglobulins (IgG, IgA, IgM) 3, 5
• 24-hour urine collection for protein electrophoresis and immunofixation (Bence Jones protein) 3, 5

Step 2: Bone Marrow Evaluation (When Indicated)

Proceed immediately to bone marrow aspiration and biopsy if laboratory screen suggests myeloma (M-protein present or abnormal free light chain ratio). 5 The bone marrow sample should be used for: 3

• Plasma cell enumeration and morphological assessment 3
• Multiparametric flow cytometry for immunophenotyping 3
• Cytogenetic/FISH studies 3

Critical technical recommendations for flow cytometry: 3

• CD38, CD138, and CD45 should all be included in at least one tube for plasma cell identification 3
• Primary gate should be based on CD38 vs. CD138 expression 3
• Minimal panel for detecting abnormal plasma cells should include CD19 and CD56 3
• Preferred panel would also include CD20, CD117, CD28, and CD27 3

Step 3: Imaging Studies

Whole-body low-dose CT or FDG-PET/CT is now preferred over skeletal survey for suspected myeloma, detecting 25.5% more lesions than plain radiographs. 5 If these advanced modalities are unavailable, obtain a skeletal bone survey including spine, pelvis, skull, humeri, and femurs. 3

MRI provides greater detail and is recommended whenever spinal cord compression is suspected. 3

Distinguishing Malignant from Normal Plasma Cells

Immunophenotypic Differences

Normal plasma cells: CD19+/CD56-/CD117-/CD20-/CD28- with polyclonal light chain expression 2

Malignant plasma cells: 2

• CD19- (95% of cases)
• CD56+ (75% of cases)
• CD117+ (30% of cases)
• CD20+ (30% of cases)
• CD28+ (15-45% of cases)
• Monoclonal kappa or lambda light chain restriction

Prognostic Significance of Plasma Cell Phenotyping

The ratio of abnormal to normal plasma cells in bone marrow is the single most useful prognostic factor for predicting progression from MGUS and smoldering myeloma to active disease. 4 This affects approximately 30% of patients whose outcome cannot be predicted from presentation features alone. 4 Patients with higher proportions of phenotypically abnormal plasma cells have significantly increased risk of progression. 4

Treatment Decisions Based on Disease Stage

MGUS Management

Immediate treatment is not recommended for patients with MGUS. 3, 4 However, follow MGUS patients every 3-6 months with M-protein quantification and complete blood count to monitor for progression. 5

Smoldering Myeloma Management

Immediate treatment is not recommended at present for patients with smoldering myeloma. 3 These patients require closer monitoring than MGUS patients due to their higher progression risk. 4

Symptomatic Myeloma Management

Treatment should be initiated in all patients with active myeloma fulfilling the CRAB criteria. 3, 4 Delaying treatment in patients with clear CRAB criteria leads to increased morbidity and mortality. 4

For elderly patients with symptomatic myeloma, bortezomib-melphalan-prednisone (VMP) for 8-12 cycles is the recommended standard. 4 The NCCN emphasizes that the best management for any cancer patient is enrollment in a clinical trial, with participation especially encouraged. 3

Minimal Residual Disease Monitoring

Quantitative flow cytometry assessment of residual abnormal plasma cells predicts treatment efficacy and outcome. 4 This requires high-sensitivity detection of abnormal plasma cells identified by immunophenotype and cytoplasmic κ/λ, expressed as percentage of total leukocytes or total plasma cells. 3, 4

Flow cytometry is suitable for demonstrating stringent complete remission, defined by the International Myeloma Working Group. 3

Critical Pitfalls to Avoid

Do not confuse severe osteoporosis with myeloma: Severe osteoporosis can cause vertebral compression fractures similar to myeloma. Always check for M-protein before assuming osteoporosis in patients with multiple vertebral fractures. 5

Do not miss early myeloma in MGUS patients: Regular monitoring is essential, as MGUS progresses at 1% per year. 5

Do not order unnecessary bone marrow biopsies: Bone marrow biopsy is mandatory only when M-protein is detected or CRAB criteria are present. 5

Recognize sample quality issues: Discrepancies between plasma cell percentages detected by flow cytometry versus morphology are primarily related to sample quality. Use first-pull aspirate samples for immunophenotyping to reduce inconsistency. 3 It is important to determine that marrow elements are present in follow-up samples, particularly normal plasma cells in minimal residual disease-negative cases. 3

Genetic Complexity and Disease Biology

Multiple myeloma patients can be stratified into two subgroups based on genomic aberrations: 3

• Hyperdiploid karyotype (40-60% of patients): Characterized by multiple trisomies with 47-75 chromosomes and rare translocations 3
• Non-hyperdiploid karyotype: Characterized by <46 chromosomes with frequent translocations 3

The 2022 International Consensus Classification formally subdivides MM into cytogenetic groups, recognizing the importance of genetics for clinical features and prognosis: MM with CCND family translocations, MM with MAF family translocations, MM with NSD2 translocation, MM with hyperdiploidy, and MM not otherwise specified. 6

Key Pathophysiological Mechanisms

Interleukin-6 is the most crucial cytokine involved in MM growth, exerting both proliferative and anti-apoptotic effects by activating osteoclastogenesis and inhibiting osteoblasts, leading to bone loss and osteolytic lesions. 3 Neoplastic plasma cells stimulate bone marrow angiogenesis through production of vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF). 3

Advanced disease with focal lesions is characterized by increased angiogenesis, while end-stage disease features highly proliferative plasmablasts that can cause extramedullary dissemination or plasma cell leukemia. 3