Molecular evidence of transient therapeutic effectiveness of natalizumab despite high-titre neutralizing antibodies

Natalizumab is a humanized monoclonal antibody directed against the alpha-4 integrin subunit of very late activation antigen-4 (VLA-4). Natalizumab neutralizing antibodies (NAB) have been found to significantly reduce beneficial effects of natalizumab treatment in multiple sclerosis. We investigated interactions of NAB with natalizumab by serial measurements of alpha-4 integrin levels on peripheral blood mononuclear cells using flow cytometry. In addition, serum concentrations of soluble vascular cell adhesion molecule-1 (sVCAM-1), the endothelial ligand of VLA-4, and serum NAB were serially determined. Natalizumab infusion led to a transient reduction in alpha-4 integrin levels on immune cells and serum sVCAM-1 levels along with serum negativity of NAB lasting for a few days post-infusion. Apparently, the high-dose effect of freshly infused natalizumab resulted in a transient neutralization of NAB possibly involving a transient therapeutic effectiveness.     

Altered CD4+/CD8+ T-cell ratios in cerebrospinal fluid of natalizumab-treated patients with multiple sclerosis

BACKGROUND: Treatment with natalizumab, a monoclonal antibody against the adhesion molecule very late activation antigen 4, an alpha4beta(1) integrin, was recently associated with the development of progressive multifocal leukoencephalopathy, a demyelinating disorder of the central nervous system caused by JC virus infection. OBJECTIVE: To test the effect of natalizumab treatment on the CD4(+)/CD8(+) T-cell ratios in cerebrospinal fluid (CSF) and peripheral blood. DESIGN: Prospective longitudinal study. SETTING: Academic and private multiple sclerosis centers. PATIENTS: Patients with multiple sclerosis (MS) treated with natalizumab, untreated patients with MS, patients with other neurologic diseases, and human immunodeficiency virus-infected patients. MAIN OUTCOME MEASURES: CD4(+) and CD8(+) T cells were enumerated in CSF and peripheral blood. The mean fluorescence intensity of unbound alpha4 integrin on peripheral blood CD4(+) and CD8(+) T cells was analyzed before and after natalizumab therapy. RESULTS: Natalizumab therapy decreased the CSF CD4(+)/CD8(+) ratio of patients with MS to levels similar to those of human immunodeficiency virus-infected patients. CD4(+)/CD8(+) ratios in peripheral blood in patients with MS progressively decreased with the number of natalizumab doses, but they remained within normal limits. Six months after the cessation of natalizumab therapy, CSF CD4(+)/CD8(+) ratios normalized. The expression of unbound alpha4 integrin on peripheral blood T cells decreases with natalizumab therapy and was significantly lower on CD4(+) vs CD8(+) T cells. CONCLUSIONS: Natalizumab treatment alters the CSF CD4(+)/CD8(+) ratio. Lower expression of unbound alpha4 integrin on CD4(+) T cells is one possible mechanism. These results may have implications for the observation that some natalizumab-treated patients with MS developed progressive multifocal leukoencephalopathy.     

Novel mechanisms of immune modulation of natalizumab in multiple sclerosis patients

The goal of this study was to investigate the effects of natalizumab therapy on the immune cell composition and phenotype in the blood of relapsing MS patients treated over the course of 12 months. We collected peripheral blood from 26 RRMS patients before treatment onset, and then 6 and 12 months after therapy. PBMC was isolated and then analyzed for phenotypic characteristics by FACS and for cytokine production by ELISA. The results of our studies showed changes in both numbers and activation states of immune cells following therapy. These changes were observed at the 6 month timepoint and generally persisted through the 12 month timepoint. The proportions of NK cells (CD3?CD56+) and hematopoetic stem cells (CD34+lin?) were increased after natalizumab treatment. Decreases were noted in numbers of CD14+ monocytes, and possibly their migratory potential, since their expression levels of α4β1 were decreased. Relative numbers of CD20+ B cells were increased, but the proportion of CD20+ cells expressing high levels of α4β1 integrin was decreased. While proportions of CD4+ and CD8+ T cells did not change, the percentage of cells expressing α4β1 integrin was significantly decreased for both subsets. Natalizumab therapy produces a number of phenotypic changes in the immune composition of peripheral blood. These changes may help to explain both the mechanisms of action of natalizumab and also shed light on the potential for the observed increase in PML in these patients.     

Differential Effects of Fingolimod and Natalizumab on B Cell Repertoires in Multiple Sclerosis Patients

Natalizumab and fingolimod are effective multiple sclerosis (MS) therapies that disrupt lymphocyte migration but have differential effects on B cell maturation and trafficking. We investigated their effects on peripheral blood (PB) and cerebrospinal fluid (CSF) B cell repertoires using next-generation deep sequencing. Paired CSF and PB B cell subsets (naïve, CD27⁺ memory, and CD27⁻IgD⁻ double-negative B cells and plasmablasts) were collected by applying flow cytometry at baseline and after 6 months of treatment and their respective heavy-chain variable region repertoires assessed by Illumina MiSeq. Treatment with fingolimod contracted, whereas natalizumab expanded circulating PB B cells. CSF B cell numbers remained stable following fingolimod treatment but decreased with natalizumab therapy. Clonal overlap between CSF and PB B cells was reduced with natalizumab treatment but remained stable with fingolimod therapy. Lineage analyses of pre- and posttreatment CSF B cell repertoires revealed large, clonally expanded B cell clusters in natalizumab-treated MS patients but no intrathecal clonal expansion following fingolimod therapy. Our findings suggest that natalizumab diminishes the exchange of peripheral and intrathecal B cells without impacting intrathecal clonal expansion. In contrast, fingolimod treatment fails to alter blood–brain barrier B cell exchange but diminishes intrathecal clonal expansion. Sphingosine-1 phosphate receptor inhibition may alter intrathecal B cell biology in MS.Supplementary InformationThe online version contains supplementary material available at 10.1007/s13311-020-00975-7.Key Words: Natalizumab, fingolimod, cerebrospinal fluid, B cell migration, B cell maturation, mass sequencing.     

B‐cell very late antigen‐4 deficiency reduces leukocyte recruitment and susceptibility to central nervous system autoimmunity

Natalizumab, which binds very late antigen‐4 (VLA‐4), is a potent therapy for multiple sclerosis (MS). Studies have focused primarily upon its capacity to interfere with T‐cell migration into the central nervous system (CNS). B cells are important in MS pathogenesis and express high levels of VLA‐4. Here, we report that the selective inhibition of VLA‐4 expression on B cells impedes CNS accumulation of B cells, and recruitment of Th17 cells and macrophages, and reduces susceptibility to experimental autoimmune encephalomyelitis. These results underscore the importance of B‐cell VLA‐4 expression in the pathogenesis of CNS autoimmunity and provide insight regarding mechanisms that may contribute to the benefit of natalizumab in MS, as well as candidate therapeutics that selectively target B cells. Ann Neurol 2015;77:902–908     

Dynamics of immune cell trafficking in interferon-beta treated multiple sclerosis patients

PURPOSE: To investigate the effects of interferon-beta-1a (IFN-beta-1a) on the trafficking of cell populations in peripheral blood cells of multiple sclerosis (MS) patients. METHODS: In this open-label pharmacodynamic study, peripheral blood was obtained from 10 relapsing-remitting (RR) MS patients just prior to and at 1, 2, 4, 8, 24, 48, 120, and 168 h after intramuscular injection of 30-microg IFN-beta-1a. Timed samples were also obtained from five controls at 0, 8, 24, 48 and 168 h. The blood cells were analyzed using four-color flow cytometry with antibody conjugates directed against cell surface proteins specific for T cells, B cells, NK cells, and the activation marker, CD69. RESULTS: IFN-beta-1a treatment resulted in selective, time-dependent effects on many cell populations in peripheral blood. The trafficking of T-helper and T-suppressor/cytotoxic subsets of T cells were qualitatively different. The most prominent effects were on the trafficking of natural killer cells, the levels of which decreased to 23.5% of pretreatment values at 8 h after treatment. The levels of CD69-positive NK cells increased to a peak value of 606% of pretreatment levels at the 24-h time point. In untreated controls, these characteristic trafficking effects were not observed. There was inter-patient heterogeneity in the levels of activated NK cells at the 6-month time point that may potentially be relevant for individualizing IFN-beta therapy. CONCLUSIONS: IFN-beta treatment can induce specific, selective, and time-dependent trafficking of cells and its effects on different subsets of a given cell type are not qualitatively similar. The dynamics indicate that the activation of NK cells by IFN-beta is possibly dependent on the trafficking of NK cells. The activated NK cell levels after prolonged therapy may potentially provide a surrogate marker for IFN-beta exposure.     

Immune surveillance in multiple sclerosis patients treated with natalizumab

OBJECTIVE: Our objective was to test whether natalizumab, an antibody against very late activating antigen (VLA)-4, interferes with central nervous system immune surveillance as assessed by leukocyte cell numbers and cellular phenotypes in cerebrospinal fluid (CSF) and peripheral blood. METHODS: Cell numbers and cellular phenotypes in CSF and peripheral blood were analyzed in multiple sclerosis (MS) patients treated with natalizumab, untreated MS patients, and patients with other neurological disease (OND). JC virus DNA in the CSF and peripheral blood was quantified by kinetic polymerase chain reaction. RESULTS: CSF leukocyte counts, CD4(+) and CD8(+) T cells, CD19(+) B cells, and CD138(+) plasma cells were significantly lower in natalizumab-treated MS patients compared with OND patients and untreated MS patients. JC virus DNA was not detected in CSF or peripheral blood from natalizumab-treated patients. Six months after cessation of natalizumab therapy, low lymphocyte counts in the CSF persisted. The patient with the highest total leukocyte and CD4(+) and CD8(+)T-cell counts in the CSF experienced a clinical relapse. INTERPRETATION: These data suggest that natalizumab treatment results in a prolonged decrease of lymphocytes in the CSF and are consistent with the hypothesis that natalizumab impairs immune surveillance of the central nervous system.     

Reactivation of JC virus and development of PML in patients with multiple sclerosis

The attention of researchers and clinicians specializing in both multiple sclerosis (MS) and JC virus (JCV), the etiologic agent of progressive multifocal leukoencephalopathy (PML), was rekindled by the development of PML in two patients with MS enrolled in a clinical trial of combination therapy with natalizumab (Tysabri) and interferon beta-1A (Avonex) in recent years. PML had not been previously reported with either MS or treatment with interferon beta alone. This occurrence of PML with alpha4beta1-integrin inhibition in MS raised a number of issues in terms both of the scientific understanding of these diseases and for the future of immunomodulatory treatment for MS. In this review, we examine the current status of knowledge of the virus, its molecular biology, life cycle, and pathogenetic mechanisms, and how this relates to the basic science and clinical perspectives of MS. A better understanding of the specific steps from JCV infection to the development of PML is key to this issue. Other critical issues for further investigation include the role of alpha4beta1-integrin inhibition by natalizumab in the re-expression of JCV from latent sites and in the inhibition of entry into the brain and peripheral sites.     

An open-label safety and drug interaction study of natalizumab (Antegren) in combination with interferon-beta (Avonex) in patients with multiple sclerosis

In this open-label drug-interaction trial, we studied 38 patients with relapsing-remitting multiple sclerosis (MS) who received 3.0 or 6.0 mg/kg of natalizumab as a single intravenous (i.v.) infusion during stable treatment with intramuscular (i.m.) interferon beta-1a 30 microg (IFNbeta-1a; Avonex). To assess the pharmacokinetic (PK) interaction of natalizumab and IFNbeta-1a, serum concentration-time data for both agents were collected and analysed. Biologic response markers of IFNbeta-1a activity, beta2-microglobulin and neopterin, were also assessed to determine effects of natalizumab on IFNbeta-1a pharmacodynamics (PD). Further, safety and immunogenicity were evaluated. The combination of drug therapies was well tolerated. Although natalizumab serum concentrations (and corresponding PK exposure measures) appeared to be somewhat elevated in the presence of IFNbeta-1a, when compared to the same dose (6.0 mg/kg) administered alone in a concurrent comparator study, the differences were generally small and unlikely to be clinically relevant. In general, natalizumab had no apparent clinically relevant effects on the PK or PD properties of IFNbeta-1a. The presence of antibodies to IFNbeta-1a and natalizumab was relatively low. Overall, the study provided safety, immunogenicity, PK and PD data to support a combination strategy for the use of natalizumab and IFNbeta-1a in the treatment of patients with relapsing-remitting MS. A large clinical study is currently in progress to evaluate the efficacy and long-term safety of this combination drug therapy.     

Dual-label immunocytochemistry of the active multiple sclerosis lesion: major histocompatibility complex and activation antigens

Fresh-frozen autopsy material containing active inflammatory lesions from 9 different patients with multiple sclerosis (MS) was analyzed by immunocytochemistry using a panel of monoclonal antibodies, and a dual-label immunocytochemical method was developed which permitted the simultaneous detection of two different surface markers on a single cell. We now report the following. (1) The predominant T-cell phenotype within MS lesions is CD2,3,8. This phenotype marks the suppressor-cytotoxic subset. (2) These cells do not express the natural killer cell marker NKH-1, which is present on a subset of CD8-positive cells in peripheral blood. (3) The infiltrating cell expresses class I (HLA A, B, C), but not class II (DR and DQ), major histocompatibility complex (MHC) molecules. (4) Other T-cell surface molecules, including the activation antigens interleukin-2 receptor, Ta1, and T11-3, as well as the marker 2H4, are largely not expressed. (5) Endothelial cells express both class I and class II MHC molecules and the 4B4 molecule in both MS and control tissue. (6) Astrocytes within the vicinity of MS lesions are predominantly class II MHC-negative. These results demonstrate that the T-cell infiltrate present in MS tissue on autopsy has a restricted phenotypic profile, but they also raise the possibility that, within this population, few activated effector cells are present.     

The effects of natalizumab on the innate and adaptive immune system in the central nervous system

Multiple sclerosis (MS) is an inflammatory demyelinating disorder of the central nervous system (CNS). Natalizumab ((R)Tysabri) is a humanized recombinant monoclonal antibody that binds to the alpha (alpha)(4) chain of the alpha(4) beta (beta)(1) integrin (very late activation antigen 4; VLA-4), and alpha(4)beta(7) integrin. Recently, two patients with MS and one patient with Crohn's disease who were treated with natalizumab in the setting of clinical trials developed progressive multifocal leukoencephalopathy (PML), an opportunistic infection of the brain with the polyoma virus JC. We recently showed that natalizumab decreases the numbers of CD4(+) and CD8(+) T lymphocytes, CD19(+) B cells, and CD138(+) plasma cells in the cerebrospinal fluid (CSF) of patients with MS on natalizumab therapy. In addition, we demonstrated that the cell numbers in CSF remained unchanged even 6 months after cessation of natalizumab treatment.     

What happens when natalizumab therapy is stopped?

Natalizumab is an α-4 integrin antagonist used for the treatment of relapsing multiple sclerosis (MS). Concerns with the drug have a risen owing to a heightened risk of progressive multifocal leukoencephalopathy, which has caused some physicians to interrupt or stop treatment altogether. The article under review evaluates the safety of natalizumab treatment interruption, including the rate and magnitude of the return of MS disease activity toward baseline levels by clinical and MRI measures. The investigators found that by 4-7 months after natalizumab treatment interruption, MS disease activity began to reach baseline levels, which is consistent with the known elimination kinetics of natalizumab. The duration of prior natalizumab exposure or alternate MS treatments during interruption was demonstrated to not affect return of disease activity. Despite nearly similar disease activity after natalizumab treatment, patients with highly active disease prior to treatment had a return of disease activity that was greater in magnitude when compared with those with less active disease. Most significantly, the study did not show evidence of rebound following natalizumab cessation. We agree with these conclusions, but note that a subgroup of MS patients may demonstrate highly active disease after natalizumab cessation.     

Treatment of multiple sclerosis with gamma interferon: exacerbations associated with activation of the immune system

We treated 18 clinically definite relapsing-remitting MS patients with recombinant gamma interferon in a pilot study designed to evaluate toxicity and dosage. Patients received low (1 microgram), intermediate (30 micrograms), or high (1,000 micrograms) doses of interferon by intravenous infusion twice a week for 4 weeks. Serum levels of gamma interferon were proportional to dose and no interferon was detected in CSF. Seven of the 18 patients had exacerbations during treatment, a significant increase compared with the prestudy exacerbation rate (p less than 0.01). Exacerbations occurred in all three dosage groups and were not precipitated by fever or other dose-dependent side effects. There were significant increases in circulating monocytes bearing class II (HLA-DR) surface antigen, in the proliferative responses of peripheral blood leukocytes, and in natural killer cell activity. These results show that systemic administration of gamma interferon has pronounced effects on cellular immunity in MS and on disease activity within the CNS, suggesting that the attacks induced during treatment were immunologically mediated. Gamma interferon is unsuitable for use as a therapeutic agent in MS. Agents that specifically inhibit gamma interferon production or counteract its effects on immune cells should be investigated as candidates for experimental therapy.     

Additional efficacy endpoints from pivotal natalizumab trials in relapsing-remitting MS

Standard clinical endpoints in multiple sclerosis (MS) studies, such as disability progression defined by the expanded disability status scale (EDSS) and annualized relapse rate, may not fully reflect all aspects of therapeutic benefit experienced by patients. Pivotal studies showed that natalizumab is effective both as monotherapy (AFFIRM study) and in combination with interferon beta-1a (IFNβ-1a) (SENTINEL study) in patients with relapsing MS. We present AFFIRM and SENTINEL data demonstrating the efficacy of natalizumab on prespecified tertiary endpoints, including extent of confirmed change in EDSS score from baseline, time to sustained progression to EDSS milestone scores, hospitalizations, corticosteroid use, and time to confirmed progression of cognitive deficits. Natalizumab significantly reduced changes in EDSS scores (P < 0.001) and proportion of patients progressing to an EDSS score ≥4.0 (P < 0.001) and ≥6.0 (P = 0.002) compared with placebo. Natalizumab + IFNβ-1a significantly reduced changes in EDSS scores compared with placebo + IFNβ-1a (P = 0.011). Based on 0.5 standard deviation change in paced auditory serial addition test-3 score, natalizumab treatment reduced the risk of confirmed progression of cognitive deficits by 43% compared with placebo (HR 0.57 [95% CI 0.37, 0.89], P = 0.013); however, no significant difference between groups was seen in SENTINEL. Natalizumab, both as monotherapy and in combination with IFNβ-1a, significantly reduced the annualized rate of MS-related hospitalizations (by 64 and 61%, respectively) and the annualized rate of relapses severe enough to require steroid treatment (by 69 and 61%, respectively) compared with placebo and placebo + IFNβ-1a (P < 0.001). These analyses underline beneficial effects of natalizumab therapy in relapsing MS patients.     

Astrocytes are not susceptible to lysis by natural killer cells

A cell population from the central nervous system, of mainly astrocytes, was prepared and their susceptibility to natural killer cell lysis was compared to YAC-1 cells and L929 cells. Two populations of natural killer cells were used: the splenocytes of mice immunized with Semliki Forest virus 3 days previously and day 5 inflammatory cells from West Nile virus-infected CBA/H mouse brain. Both cell populations showed high lytic activity on YAC-1 cells and the effector cells possessed conventional natural killer cell markers, i.e., Thy1 +/-, L3T4-, Lyt2-, GM1+. The results revealed that astrocytes are not susceptible to lysis by natural killer cells and hence argue against the hypothesis that the absolute concentration of MHC antigens on the cell surface determines the susceptibility to natural killer cell lysis.     

Recurrence or rebound of clinical relapses after discontinuation of natalizumab therapy in highly active MS patients

A number of studies have reported flare-up of multiple sclerosis (MS) disease activity after cessation of natalizumab, increasing to a level beyond the pre-natalizumab treatment level. Our aim was to describe the development in clinical disease activity following cessation of natalizumab therapy in a large unselected cohort of highly active patients. We studied 375 highly active patients who had suffered at least two significant relapses within 1 year or three relapses within 2 years, or had been treated with mitoxantrone for highly active disease. All patients had discontinued therapy with natalizumab after at least 24 weeks on therapy, and had been followed 3–12 months (mean 8.9 months) after cessation of natalizumab therapy. The annualised relapse rate before start of natalizumab therapy was 0.94 (95 % confidence interval [CI] 0.88–1.00), 0.47 (95 % CI 0.43–0.52) during natalizumab therapy, 0.63 (95 % CI 0.51–0.76) 1–6 months after natalizumab and 0.55 (95 % CI 0.42–0.70) 7–12 months after natalizumab. However, 83 (22 %) of the patients could be classified as showing rebound of relapses, defined as a higher individual relapse rate after cessation of natalizumab than before natalizumab. These patients had a higher annualised relapse rate during natalizumab therapy. For the whole patient group, the relapse rate after discontinuation did not exceed the pre-natalizumab relapse rate at any time, but 22 % of the patients showed rebound of relapses after discontinuation of natalizumab.     

Expansion of CD4+CD28- T cells producing high levels of interferon-{gamma} in peripheral blood of patients with multiple sclerosis

CD4(+) T cells that lack surface expression of the CD28 co-stimulatory molecule (CD4(+)CD28(-) T cells) were expanded in peripheral blood of patients with multiple sclerosis (MS) [5.20 +/- 1.67% vs 13.00 +/- 2.68% (healthy controls (HC) versus patients with MS)]. Both the CD4(+)CD28(+) and CD4(+)CD28(-) T-cell populations of patients with MS produced higher levels of interferon (IFN)-gamma compared with those in HC. In particular, the proportion of IFN-gamma(+) cells among CD4(+)CD28(-) T cells from patients with MS was considerably high. However, expression of co-stimulatory molecules including inducible costimulator (ICOS), activating natural killer receptors, or members of tumor necrosis factor receptor family that replace CD28 in CD4(+)CD28(-) T cells of patients with MS could not be identified. A unique subpopulation bearing the CD45RA(high)CCR7(-) phenotype was identified among the CD4(+)CD28(-) T cells of some patients with MS. Because only MS samples contained this CD45RA(high)CCR7(-) population attributed to terminally differentiated effector memory cells and lacked naive CD45RA(high)CCR7(+) cells, we suggest that CD4(+)CD28(-) T cells of patients with MS represent a cell population which is in more differentiated state than healthy subjects. In patients treated with IFN-beta-1b, IFN-gamma production from CD4(+)CD28(+) T cells was suppressed compared with that in untreated patients. On the contrary, in the CD4(+)CD28(-) population, production of IFN-gamma in IFN-beta-1b-treated patients was not significantly suppressed compared with that in untreated patients with MS. Thus, an additional treatment strategy that specifically targets this cell population may enhance the beneficial effect of IFN-beta on MS.     

Natalizumab: targeting alpha4-integrins in multiple sclerosis

In 1992, it was shown that monoclonal antibodies blocking alpha(4)-integrins prevent the development of experimental autoimmune encephalomyelitis, an animal model for multiple sclerosis (MS). As alpha(4)beta(1)-integrin was demonstrated to mediate the attachment of immune-competent cells to inflamed brain endothelium in experimental autoimmune encephalomyelitis, the therapeutic effect was attributed to the inhibition of immune cell extravasation and inflammation in the central nervous system. This novel therapeutic approach was rapidly and successfully translated into the clinic. The humanized anti-alpha(4)-integrin antibody natalizumab demonstrated an unequivocal therapeutic effect in preventing relapses and slowing down the pace of neurological deterioration in patients with relapsing-remitting MS in phase II and phase III clinical trials. The occurrence of 3 cases of progressive multifocal leukoencephalopathy in patients treated with natalizumab led to the voluntary withdrawal of the drug from the market. After a thorough safety evaluation of all patients receiving this drug in past and ongoing studies for MS and Crohn's disease, natalizumab again obtained approval in the US and the European Community. A treatment targeting leukocyte trafficking in MS has now re-entered the clinic. Further thorough evaluation is necessary for a better understanding of the risk-benefit balance of this new treatment option for relapsing MS. In this review, we discuss the basic mechanism of action, key clinical results of clinical trials and the emerging indication of natalizumab in MS.     

Circulating natural killer cell phenotypes in men and women with major depression. Relation to cytotoxic activity and severity of depression.

The effects of major depression on peripheral blood natural killer cell phenotypes and natural killer cell activity were studied by comparing depressed and normal control subjects. Depressed subjects exhibited (1) significant reductions in Leu-11 (CD16) natural killer effector cells and natural killer cell activity and (2) a dissociation of the normal positive correlation between the percentage of Leu-11 cells and natural killer cell activity. These findings suggest that alterations in the availability and the killing capacity of circulating Leu-11 natural killer cells appear to be responsible for depression-related reductions in natural killer cell activity. Moreover, men with major depression showed marked reductions in Leu-11 cells, natural killer cell activity, and Leu-7 (HNK-1) lymphocytes compared with normal control men. By contrast, depressed women did not differ significantly from normal control women on any of these three immune function measures. Severity of depression as assessed by Hamilton Rating Scale for Depression scores was not associated with natural killer cell activity or Leu-7 lymphocyte levels in either men or women with major depression. Hamilton Rating Scale for Depression severity ratings were, however, strongly inversely correlated with Leu-11 lymphocyte counts among men, but not women, with major depression. These data begin to elucidate the immunological mechanisms by which natural killer cell activity is altered in depression and suggest that some measures of immunity may be differentially affected in male and female subjects with the syndrome of major depression.