Chiung Yu Huang

The Plasmodium falciparum-Specific Human Memory B Cell Compartment Expands Gradually with Repeated Malaria Infections

Immunity to Plasmodium falciparum (Pf) malaria is only acquired after years of repeated infections and wanes rapidly without ongoing parasite exposure. Antibodies are central to malaria immunity, yet little is known about the B-cell biology that underlies the inefficient acquisition of Pf-specific humoral immunity. This year-long prospective study in Mali of 185 individuals aged 2 to 25 years shows that Pf-specific memory B-cells and antibodies are acquired gradually in a stepwise fashion over years of repeated Pf exposure. Both Pf-specific memory B cells and antibody titers increased after acute malaria and then, after six months of decreased Pf exposure, contracted to a point slightly higher than pre-infection levels. This inefficient, stepwise expansion of both the Pf-specific memory B-cell and long-lived antibody compartments depends on Pf exposure rather than age, based on the comparator response to tetanus vaccination that was efficient and stable. These observations lend new insights into the cellular basis of the delayed acquisition of malaria immunity.

Joint modeling and estimation for recurrent event processes and failure time data

Recurrent event data are commonly encountered in longitudinal follow-up studies related to biomedical science, econometrics, reliability, and demography. In many studies, recurrent events serve as important measurements for evaluating disease progression, health deterioration, or insurance risk. When analyzing recurrent event data, an independent censoring condition is typically required for the construction of statistical methods. In some situations, however, the terminating time for observing recurrent events could be correlated with the recurrent event process, thus violating the assumption of independent censoring. In this article, we consider joint modeling of a recurrent event process and a failure time in which a common subject-specific latent variable is used to model the association between the intensity of the recurrent event process and the hazard of the failure time. The proposed joint model is flexible in that no parametric assumptions on the distributions of censoring times and latent variables are made, and under the model, informative censoring is allowed for observing both the recurrent events and failure times. We propose a “borrow-strength estimation procedure” by first estimating the value of the latent variable from recurrent event data, then using the estimated value in the failure time model. Some interesting implications and trajectories of the proposed model are presented. Properties of the regression parameter estimates and the estimated baseline cumulative hazard functions are also studied.

Half- vs Full-Dose Trivalent Inactivated Influenza Vaccine (2004-2005) Age, Dose, and Sex Effects on Immune Responses

BACKGROUND Optimal public health strategies for managing influenza vaccine shortages are not yet defined. Our objective was to determine the effects of age, sex, and dose on the immunogenicity of intramuscular trivalent inactivated vaccine (TIV). METHODS Healthy adults aged 18 to 64 years, stratified by age (18-49 and 50-64 years) and sex, were randomized to receive full- or half-dose TIV. Hemagglutination inhibition titers against vaccine antigens were measured before and 21 days after immunization. A primary outcome of noninferiority was defined as a difference of less than 20% in the upper 95% confidence interval (CI) of the proportion of subjects with strain-specific hemagglutination inhibition antibody titers of 1:40 or higher after vaccination. Secondary outcomes included geometric mean titers, after vaccination side effects, and occurrences of influenza-like illnesses. RESULTS Among previously immunized subjects (N = 1114) receiving half- vs full-dose TIV (age, 18-49 years, n = 284 [half] and n = 274 [full]; and age 50-64 years, n = 276 [half] and n = 280 [full]), CIs for proportions of subjects with hemagglutination inhibition antibody titers of 1:40 or higher excluded substantial reduction for all antigens in the 18- to 49-year age group and for B/Shanghai/361/2002 (B) and A/Fujian/411/2002 (A/H3N2) in the 50- to 64-year age group. Geometric mean titer in the female 18- to 49-year age group exceeded male responses for all strains: responses to half-dose TIV that were comparable with male full-dose responses for A/New Caledonia/20/99 (A/H1N1) antigen, 25.4 (95% CI, 20.9-30.9) vs 25.6 (95% CI, 21.3-30.9); A/H3N2 antigen, 60.8 (95% CI, 50.8-72.7) vs 44.1 (95% CI, 37.6-51.8); and B antigen, 64.4 (95% CI, 53.9-76.9) vs 60.7 (95% CI, 51.4-71.7) (findings were similar for the 50- to 64-year age group). Some injection site and systemic reactions (myalgias and/or arthralgias [P < .05], headache [P < .001], and impact of fatigue [P < .05]) were significantly lower in men. The relative risk of medical visits and hospitalizations for influenza-like illnesses were similar in the half- and full-dose groups regardless of age. CONCLUSIONS Antibody responses to intramuscular half-dose TIV in healthy, previously immunized adults were not substantially inferior to the full-dose vaccine, particularly for ages 18 to 49 years. Significantly higher geometric mean titer responses in women were identified for all ages, regardless of dose or influenza strain. Half-dose vaccination may be an effective strategy for healthy adults younger than 50 years in the setting of an influenza vaccine shortage.

An Intensive Longitudinal Cohort Study of Malian Children and Adults Reveals No Evidence of Acquired Immunity to Plasmodium falciparum Infection

BACKGROUND In experimental models of human and mouse malaria, sterilizing liver stage immunity that blocks progression of Plasmodium infection to the symptomatic blood stage can be readily demonstrated. However, it remains unclear whether individuals in malaria-endemic areas acquire such immunity. METHODS In Mali, 251 healthy children and adults aged 4-25 years who were free of blood-stage Plasmodium infection by polymerase chain reaction (PCR) were enrolled in a longitudinal study just prior to an intense 6-month malaria season. Subsequent clinical malaria episodes were detected by weekly active surveillance and self-referral. Asymptomatic P. falciparum infections were detected by blood-smear microscopy and PCR analysis of dried blood spots that had been collected every 2 weeks for 7 months. RESULTS As expected, the risk of clinical malaria decreased with increasing age (log-rank test, P = .0038). However, analysis of PCR data showed no age-related differences in P. falciparum infection risk (log-rank test, P = .37). CONCLUSIONS Despite years of exposure to intense P. falciparum transmission, there is no evidence of acquired, sterile immunity to P. falciparum infection in this population, even as clinical immunity to blood-stage malaria is clearly acquired. Understanding why repeated P. falciparum infections do not induce sterile protection may lead to insights for developing vaccines that target the liver stage in malaria-endemic populations.

Wide Prevalence of Heterosubtypic Broadly Neutralizing Human Anti–Influenza A Antibodies

Sera from H5N1 vaccinees and IVIG were analyzed for cross-reactive Abs against HA of influenza A viruses. Data quantitatively show the presence of two populations of heterosubtypic neutralizing Abs against either Group 1 only or Group 1+2 viruses.

Sickle Cell Trait is Associated with a Delayed Onset of Malaria: Implications for Time-to-Event Analysis in Clinical Studies of Malaria

BACKGROUND The World Health Organization (WHO) recently recommended that the time to first malaria episode serve as the primary end point in phase III malaria vaccine trials--the first of which will be held in Africa. Although common red blood cell (RBC) polymorphisms such as sickle hemoglobin (HbS) are known to protect against malaria in Africa, their impact on this end point has not been investigated. METHODS A longitudinal study of 225 individuals aged 2-25 years was conducted in Mali. The association between common RBC polymorphisms and the time to first malaria episode was evaluated. RESULTS Among children aged 2-10 years, sickle cell trait (HbAS) was associated with a 34-day delay in the median time to first malaria episode (P= .017) Cox regression analysis showed that greater age (hazard ratio [HR], 0.87 [95% CI, 0.80-0.94]; (P= .001), HbAS (HR, 0.48 [95% CI, 0.26-0.91]; (P= .024), and asymptomatic parasitemia at enrollment (HR, 0.35 [95% CI, 0.14-0.85]; (P= .021) were associated with decreased malaria risk. CONCLUSION Given the delay in the time to first malaria episode associated with HbAS, it would be advisable for clinical trials and observational studies that use this end point to include Hb typing in the design of studies conducted in areas where HbAS is prevalent.

The TLR9 Ligand CpG Promotes the Acquisition of Plasmodium falciparum-Specific Memory B Cells in Malaria-Naive Individuals

Despite the central role of memory B cells (MBC) in protective immune responses, little is understood about how they are acquired in naive individuals in response to Ag exposure, and how this process is influenced by concurrent activation of the innate immune system’s TLR. In this longitudinal study of malaria-naive individuals, we examined the MBC response to two candidate malaria vaccines administered with or without CpG, a TLR9 ligand. We show that the acquisition of MBC is a dynamic process in which the vaccine-specific MBC pool rapidly expands and then contracts, and that CpG enhances the kinetics, magnitude, and longevity of this response. We observed that the percentage of vaccine-specific MBC present at the time of reimmunization predicts vaccine-specific Ab levels 14 days later; and that at steady-state, there is a positive correlation between vaccine-specific MBC and Ab levels. An examination of the total circulating MBC and plasma cell pools also suggests that MBC differentiate into plasma cells through polyclonal activation, independent of Ag specificity. These results provide important insights into the human MBC response, which can inform the development of vaccines against malaria and other pathogens that disrupt immunological memory.

In Vitro Growth-Inhibitory Activity and Malaria Risk in a Cohort Study in Mali

ABSTRACT Immunity to the asexual blood stage of Plasmodium falciparum is complex and likely involves several effector mechanisms. Antibodies are thought to play a critical role in malaria immunity, and a corresponding in vitro correlate of antibody-mediated immunity has long been sought to facilitate malaria vaccine development. The growth inhibition assay (GIA) measures the capacity of antibodies to limit red blood cell (RBC) invasion and/or growth of P. falciparum in vitro. In humans, naturally acquired and vaccine-induced P. falciparum-specific antibodies have growth-inhibitory activity, but it is unclear if growth-inhibitory activity correlates with protection from clinical disease. In a longitudinal study in Mali, purified IgGs, obtained from plasmas collected before the malaria season from 220 individuals aged 2 to 10 and 18 to 25 years, were assayed for growth-inhibitory activity. Malaria episodes were recorded by passive surveillance over the subsequent 6-month malaria season. Logistic regression showed that greater age (odds ratio [OR], 0.78; 95% confidence interval [95% CI], 0.63 to 0.95; P = 0.02) and growth-inhibitory activity (OR, 0.50; 95% CI, 0.30 to 0.85; P = 0.01) were significantly associated with decreased malaria risk in children. A growth-inhibitory activity level of 40% was determined to be the optimal cutoff for discriminating malaria-immune and susceptible individuals in this cohort, with a sensitivity of 97.0%, but a low specificity of 24.3%, which limited the assays ability to accurately predict protective immunity and to serve as an in vitro correlate of antibody-mediated immunity. These data suggest that antibodies which block merozoite invasion of RBC and/or inhibit the intra-RBC growth of the parasite contribute to but are not sufficient for the acquisition of malaria immunity.

Cryptococcosis and idiopathic CD4 lymphocytopenia.

We reviewed the cases of 11 patients with cryptococcosis and idiopathic CD4 lymphocytopenia (ICL) referred to our institution in the previous 12 years, as well as 42 similar cases reported in the literature, to assess the characteristics of the infection in this population. Cryptococcosis in 53 patients with ICL had features in common with cryptococcosis in previously normal patients. ICL patients had a slight male predominance (1.2:1) and a median age of presentation of 41 years (range, 4.5-85 yr). Initial cerebrospinal fluid findings showed glucose below 40 mg/dL in 60% of the patients, a median pleocytosis of 59 white blood cells/mm3 (range, 0-884), and protein of 156 mg/dL (range, 25-402 mg/dL). The median CD4 count at diagnosis of ICL and at the last available measurement was 82 (range, 7-292) and 132 (range, 13-892) cells/mm3, respectively, for an average follow-up of 32 months in 46 patients. Unlike previously normal patients with cryptococcosis, those with ICL had an excess incidence of dermatomal zoster (7 episodes in 46 ICL cases). Pneumocystis pneumonia was rare (1 case), casting doubt on the need for prophylaxis in patients with ICL. A favorable outcome (cured or improved) may be more common in ICL patients than in previously normal patients with cryptococcal meningitis and no predisposing factors. Identification of ICL in patients who were apparently normal before the onset of cryptococcosis appears to be useful because it predicts a favorable outcome. Patients with cryptococcal infection and ICL have an increased likelihood of developing dermatomal zoster. The long-term follow-up of these patients offers some reassurance regarding favorable prognosis. Abbreviations: 5-FC = flucytosine, AmB = amphotericin B, BAL = bronchoalveolar lavage, CI = confidence intervals, CNS = central nervous system, CSF = cerebrospinal fluid, CT = computed tomography, HIV = human immunodeficiency virus, ICL = idiopathic CD4 lymphocytopenia, MAC = Mycobacterium avium complex, MRI = magnetic resonance imaging, NIH = National Institutes of Health, PCP = Pneumocystis jirovecii pneumonia, TMPSMX = trimethoprim-sulfamethoxazole, VZV = herpes zoster, WBC = white blood cells.

Parasite Burden and Severity of Malaria in Tanzanian Children

BACKGROUND Severe Plasmodium falciparum malaria is a major cause of death in children. The contribution of the parasite burden to the pathogenesis of severe malaria has been controversial. METHODS We documented P. falciparum infection and disease in Tanzanian children followed from birth for an average of 2 years and for as long as 4 years. RESULTS Of the 882 children in our study, 102 had severe malaria, but only 3 had more than two episodes. More than half of first episodes of severe malaria occurred after a second infection. Although parasite levels were higher on average when children had severe rather than mild disease, most children (67 of 102) had high-density infection (>2500 parasites per 200 white cells) with only mild symptoms before severe malaria, after severe malaria, or both. The incidence of severe malaria decreased considerably after infancy, whereas the incidence of high-density infection was similar among all age groups. Infections before and after episodes of severe malaria were associated with similar parasite densities. Nonuse of bed nets, placental malaria at the time of a womans second or subsequent delivery, high-transmission season, and absence of the sickle cell trait increased severe-malaria risk and parasite density during infections. CONCLUSIONS Resistance to severe malaria was not acquired after one or two mild infections. Although the parasite burden was higher on average during episodes of severe malaria, a high parasite burden was often insufficient to cause severe malaria even in children who later were susceptible. The diverging rates of severe disease and high-density infection after infancy, as well as the similar parasite burdens before and after severe malaria, indicate that naturally acquired resistance to severe malaria is not explained by improved control of parasite density. (Funded by the National Institute of Allergy and Infectious Diseases and others.).