The spinal cord injury-induced immune deficiency syndrome: results of the SCIentinel study

Abstract

Infections are prevalent after spinal cord injury (SCI), constitute the main cause of death and are a rehabilitation confounder associated with impaired recovery. We hypothesize that SCI causes an acquired lesion-dependent (neurogenic) immune suppression as an underlying mechanism to facilitate infections. The international prospective multicentre cohort study (SCIentinel; protocol registration DRKS00000122; n = 111 patients) was designed to distinguish neurogenic from general trauma-related effects on the immune system. Therefore, SCI patient groups differing by neurological level, i.e. high SCI [thoracic (Th)4 or higher]; low SCI (Th5 or lower) and severity (complete SCI; incomplete SCI), were compared with a reference group of vertebral fracture (VF) patients without SCI. The primary outcome was quantitative monocytic Human Leukocyte Antigen-DR expression (mHLA-DR, synonym MHC II), a validated marker for immune suppression in critically ill patients associated with infection susceptibility. mHLA-DR was assessed from Day 1 to 10 weeks after injury by applying standardized flow cytometry procedures. Secondary outcomes were leucocyte subpopulation counts, serum immunoglobulin levels and clinically defined infections. Linear mixed models with multiple imputation were applied to evaluate group differences of logarithmic-transformed parameters. Mean quantitative mHLA-DR [ln (antibodies/cell)] levels at the primary end point 84 h after injury indicated an immune suppressive state below the normative values of 9.62 in all groups, which further differed in its dimension by neurological level: high SCI [8.95 (98.3% confidence interval, CI: 8.63; 9.26), n = 41], low SCI [9.05 (98.3% CI: 8.73; 9.36), n = 29], and VF without SCI [9.25 (98.3% CI: 8.97; 9.53), n = 41, P = 0.003]. Post hoc analysis accounting for SCI severity revealed the strongest mHLA-DR decrease [8.79 (95% CI: 8.50; 9.08)] in the complete, high SCI group, further demonstrating delayed mHLA-DR recovery [9.08 (95% CI: 8.82; 9.38)] and showing a difference from the VF controls of -0.43 (95% CI: -0.66; -0.20) at 14 days. Complete, high SCI patients also revealed constantly lower serum immunoglobulin G [-0.27 (95% CI: -0.45; -0.10)] and immunoglobulin A [-0.25 (95% CI: -0.49; -0.01)] levels [ln (g/l × 1000)] up to 10 weeks after injury. Low mHLA-DR levels in the range of borderline immunoparalysis (below 9.21) were positively associated with the occurrence and earlier onset of infections, which is consistent with results from studies on stroke or major surgery. Spinal cord injured patients can acquire a secondary, neurogenic immune deficiency syndrome characterized by reduced mHLA-DR expression and relative hypogammaglobulinaemia (combined cellular and humoral immune deficiency). mHLA-DR expression provides a basis to stratify infection-risk in patients with SCI.

Keywords: MHC class II; host defense; major histocompatibility complex class II; neurogenic immune suppression.

Figure 1

Patient selection and analysis chart and the distribution of baseline characteristics across the SCI groups and the VF group. Three patients were withdrawn from the study due to a delayed awareness of the exclusion criteria (renal-cell carcinoma, ankylosing spondylitis, HIV-infection). Six cases treated with methylprednisolone according to the NASCIS-scheme were excluded from the analysis. The analysis of the primary end point 3–4 days (84 h) after injury consisted of a comparison of (i) high SCI (neurological level Th4 or higher); (ii) low SCI (neurological level Th5 or lower); and (iii) neurologically unaffected VF (vertebral fracture without SCI) groups. Subsequently, a post hoc analysis strategy was additionally accounting for effects of the SCI severity (ASIA impairment scale – AIS), because it also defines the degree of autonomic denervation below the neurological level. For the detailed exploration, neurological (neurogenic) lesion characteristics such as SCI level and severity (complete versus incomplete lesions) were taken into account. Consequently, the SCI population was segregated into four groups comprising (i) high complete SCI (neurological level Th4 or higher, AIS A); (ii) high incomplete SCI (neurological level Th4 or higher, AIS B-D); (iii) low complete SCI (neurological level Th5 or lower, AIS A); and (iv) low incomplete SCI (neurological level Th5 or lower, AIS B-D). SCI groups were compared to the (v) VF group (vertebral fracture without SCI). Missing values: n = 1 for age in the low complete SCI group. AIS = American Spinal Injury Association (ASIA) Impairment Scale; C = cervical; IQR = interquartile range; L = lumbar; NASCIS = National Acute Spinal Cord Study; S = sacral; SCI = spinal cord injury; SCI-IDS = spinal cord injury-induced immune deficiency syndrome; SD = standard deviation; Th = thoracic; VF = vertebral fracture.

Figure 2

Spinal cord lesion-dependent (neurogenic) effects on mHLA-DR (linear mixed model). Standardized quantitative estimation of mHLA-DR expression [number of anti-HLA-DR antibodies bound per monocyte (ab/cell)], a validated marker for temporary systemic immune suppression in critically ill patients.^,,, (A) Estimated marginal means with 95% CI of mHLA-DR values [ln(ab/cell)] over time calculated for the five groups at the per-protocol time points of blood collection using linear mixed models (random intercept) adjusted for age, sex, treatment centre, time point of measure and the time point of first spine surgery after multiple imputation for missing values (30 complete datasets). The total number of actual observed mHLA-DR measurements was 402. Goodness of fit (mean of m = 30 models) conditional R2 (fixed and random effects): 0.630; marginal R2 (fixed effects): 0.354. The blue horizontal lines indicate the thresholds towards more serious stages of immune suppression; dotted line ln (<15 000 ab/cell) = ‘immune suppression'; dotted/broken line ln (<10 000 ab/cell) = ‘borderline immunoparalysis'; broken line ln (<5000 ab/cell) = ‘immunoparalysis’.^, (B) Estimated mean differences with 95% CI of ln mHLA-DR of the four SCI groups in relation to the VF group indicated as green broken line based of same regression models as for A. Reduced mHLA-DR is a characteristic hallmark of temporary immunosuppression at acute and subacute stages of the spinal cord injury-induced immune deficiency syndrome (SCI-IDS). Neuroanatomical definition of the groups: high complete SCI (neurological level Th4 or higher, AIS A); high incomplete SCI (neurological level Th4 or higher, AIS B-D); low complete SCI (neurological level Th5 or lower, AIS A); low incomplete SCI (neurological level Th5 or lower, AIS B-D); VF group (vertebral fracture without SCI). ab = antibodies; AIS = American Spinal Injury Association (ASIA) Impairment Scale; CI = confidence interval; mHLA-DR = monocytic Human Leukocyte Antigen-DR expression; ln = log-transformed; SCI = spinal cord injury; VF = vertebral fracture.

Figure 3

Fluctuation of leucocyte subpopulations. Estimated marginal means with 95% CI of (A) neutrophil, (B) monocyte, and (C) lymphocyte count [ln(cells/nl × 1000)] calculated for the five groups at the per-protocol time points of blood collection using linear mixed models (random intercept) adjusted for age, sex, treatment centre, time point of measure, and the time point of first spine surgery after multiple imputation for missing values (30 complete datasets). The total number of actual observed measurements for neutrophils, monocytes, and lymphocytes was 391, 389, and 388, respectively. For differences in estimated marginal means and measures for goodness of fit see Supplementary Table 4. The broken lines indicate the upper bound (red) and the lower bound (blue) of the reference areas. Neuroanatomical definition of the groups: high complete SCI (neurological level Th4 or higher, AIS A); high incomplete SCI (neurological level Th4 or higher, AIS B-D); low complete SCI (neurological level Th5 or lower, AIS A); low incomplete SCI (neurological level Th5 or lower, AIS B-D); VF group (vertebral fracture without SCI). AIS = American Spinal Injury Association (ASIA) Impairment Scale; CI = confidence interval; ln = log-transformed; SCI = spinal cord injury; VF = vertebral fracture.

Figure 4

Fluctuation of lymphocyte subpopulations. Estimated marginal means of (A) CD3+ T cell, (B) CD19+ B cell, and (C) CD16+ NK cell counts [ln(cell/nl × 1000)] calculated for the five groups at the per-protocol time points of blood collection using linear mixed models (random intercept) adjusted for age, sex, treatment centre, time point of measure, and the time point of first spine surgery after multiple imputation for missing values (30 complete datasets). The total number of actual observed measurements for CD3+ T cells, CD19+ B cells, and CD16+ NK cells was 397 each. For differences in estimated marginal means and measures for goodness of fit see Supplementary Table 5. The broken lines indicate the upper bound (red) and the lower bound (blue) of the reference areas. Neuroanatomical definition of the groups: high complete SCI (neurological level Th4 or higher, AIS A); high incomplete SCI (neurological level Th4 or higher, AIS B-D); low complete SCI (neurological level Th5 or lower, AIS A); low incomplete SCI (neurological level Th5 or lower, AIS B-D); VF group (vertebral fracture without SCI). AIS = American Spinal Injury Association (ASIA) Impairment Scale; CI = confidence interval; ln = log-transformed; SCI = spinal cord injury; VF = vertebral fracture.

Figure 5

Spinal cord lesion-dependent (neurogenic) effects on serum immunoglobulin levels. Estimated marginal means of (A) IgG, (B) IgA, and (C) IgM levels [ln(g/l × 1000)] calculated for the five groups at the per-protocol time points of blood collection using linear mixed models (random intercept) adjusted for age, sex, treatment center, time point of measure, and the time point of first spine surgery after multiple imputation for missing values (30 complete datasets). The total number of actual observed measurements for IgG, IgA, and IgM was 419 each. For differences in estimated marginal means and measures for goodness of fit see Supplementary Table 6. The broken red lines indicate the upper bound and the broken blue lines the lower bound of the reference areas. The high complete SCI group revealed constantly lower serum IgG and IgA levels compared to the VF group up to 10 weeks (Supplementary Table 6). Relative and absolute hypogammaglobulinaemia are symptoms contributing to the spinal cord injury-induced immune deficiency syndrome (SCI-IDS) in a lesion severity and level dependent manner. Neuroanatomical definition of the groups: high complete SCI (neurological level Th4 or higher, AIS A); high incomplete SCI (neurological level Th4 or higher, AIS B-D); low complete SCI (neurological level Th5 or lower, AIS A); low incomplete SCI (neurological level Th5 or lower, AIS B-D); VF group (vertebral fracture without SCI). AIS = American Spinal Injury Association (ASIA) Impairment Scale; CI = confidence interval; Ig = immunoglobulin; ln = log-transformed; SCI = spinal cord injury; VF = vertebral fracture.

Figure 6

SCI-IDS severity (reduced mHLA-DR) associates with acquired infections. Estimated marginal means with 95% CI of mHLA-DR values [ln number of anti-HLA-DR antibodies bound per monocyte (ab/cell)] and its association with the timing of first infections in the SCI population calculated using linear mixed models in the observed data. Numbers in brackets indicate the number of patients with available mHLA-DR measurement within each category of infection onset. Patients without infection until premature dropout were assigned to the censored group. A total of 260 mHLA-DR measurements were included in this analysis. The blue horizontal lines indicate the thresholds towards more serious stages of immune suppression; dotted line ln (<15 000 ab/cell) = immune suppression; dotted/broken line ln (<10 000 ab/cell) = ‘borderline immunoparalysis'; broken line ln (<5000 ab/cell) = ‘immunoparalysis’.^, ab = antibodies; CI = confidence interval; mHLA-DR = monocytic Human Leukocyte Antigen-DR expression; ln = log-transformed; SCI = spinal cord injury.