Isabella Quinti

Human immunoglobulin M memory B cells controlling Streptococcus pneumoniae infections are generated in the spleen.

Splenectomized and asplenic patients have a high incidence of infections by encapsulated bacteria and do not respond to polysaccharide vaccines. To understand whether the absence of the spleen is associated with a defined B cell defect, we analyzed B cell subsets in the peripheral blood. We found that a population of B cells known as immunoglobulin (Ig)M memory is lacking in patients without spleen. The absence of IgM memory B cells correlates with an impaired immune response to encapsulated bacteria not only in splenectomized patients, but also in individuals with an intact spleen. We show that the physiological and transient predisposition to pneumococcal infections of young children (0–2 yr) is associated with the lack of circulating IgM memory B cells and of serum antipolysaccharide IgM. We also demonstrate that IgM memory B cells are undetectable in a fraction of patients with common variable immunodeficiency, who have recurrent and invasive infections by encapsulated bacteria. IgM memory B cells, therefore, require the spleen for their generation and/or survival and are responsible for the protection against encapsulated bacteria.

LONG-TERM FOLLOW-UP AND OUTCOME OF A LARGE COHORT OF PATIENTS WITH COMMON VARIABLE IMMUNODEFICIENCY

Common Variable Immunodeficiency belongs to the group of rare diseases encompassing antibody deficiency syndromes of highly variable clinical presentation and outcome. The multicenter prospective study on a cohort of 224 patients with Common Variable Immunodeficiency provides an updated view of the spectrum of illnesses which occurred at the clinical onset and over a long period of follow-up (mean time: 11 years) and information on the effects of long-term immunoglobulin treatment. The mean age at the time of diagnosis was 26.6 years. Seventy-five patients were younger than 14 years of age. The mean age at the onset of symptoms was 16.9 years. This implicates with a mean diagnostic delay of 8.9 years. Respiratory tract infections were the most prominent clinical problem observed at diagnosis and during the follow-up. Intravenous immunoglobulin administration induced a significant reduction in the incidence of acute infections, mainly acute pneumonia and acute otitis. However, a progressive increase in the prevalence of patients with chronic diseases, mainly sinusitis and lung disease, was observed in all age groups, including the pediatric population. The morbidity of Common Variable Immunodeficiency due to all associated clinical conditions increased over time despite an adequate replacement with intravenous immunoglobulins. Our data stressed the need to develop international guidelines for the prevention and therapy of chronic lung disease, chronic sinusitis, chronic diarrhoea, and chronic granulomatosis in patients with humoral immunodeficiencies.

CpG Drives Human Transitional B Cells to Terminal Differentiation and Production of Natural Antibodies

The receptor TLR9, recognizing unmethylated bacterial DNA (CpG), is expressed by B cells and plays a role in the maintenance of serological memory. Little is known about the response of B cells stimulated with CpG alone, without additional cytokines. In this study, we show for the first time the phenotypic modification, changes in gene expression, and functional events downstream to TLR9 stimulation in human B cell subsets. In addition, we demonstrate that upon CpG stimulation, IgM memory B cells differentiate into plasma cells producing IgM Abs directed against the capsular polysaccharides of Streptococcus pneumoniae. This novel finding proves that IgM memory is the B cell compartment responsible for the defense against encapsulated bacteria. We also show that cord blood transitional B cells, corresponding to new bone marrow emigrants, respond to CpG. Upon TLR9 engagement, they de novo express AID and Blimp-1, genes necessary for hypersomatic mutation, class-switch recombination, and plasma cell differentiation and produce Abs with anti-pneumococcal specificity. Transitional B cells, isolated from cord blood, have not been exposed to pneumococcus in vivo. In addition, it is known that Ag binding through the BCR causes apoptotic cell death at this stage of development. Therefore, the ability of transitional B cells to sense bacterial DNA through TLR9 represents a tool to rapidly build up the repertoire of natural Abs necessary for our first-line defense at birth.

Circulating CD21low B cells in common variable immunodeficiency resemble tissue homing, innate-like B cells.

The homeostasis of circulating B cell subsets in the peripheral blood of healthy adults is well regulated, but in disease it can be severely disturbed. Thus, a subgroup of patients with common variable immunodeficiency (CVID) presents with an extraordinary expansion of an unusual B cell population characterized by the low expression of CD21. CD21low B cells are polyclonal, unmutated IgM+IgD+ B cells but carry a highly distinct gene expression profile which differs from conventional naïve B cells. Interestingly, while clearly not representing a memory population, they do share several features with the recently defined memory-like tissue, Fc receptor-like 4 positive B cell population in the tonsils of healthy donors. CD21low B cells show signs of previous activation and proliferation in vivo, while exhibiting defective calcium signaling and poor proliferation in response to B cell receptor stimulation. CD21low B cells express decreased amounts of homeostatic but increased levels of inflammatory chemokine receptors. This might explain their preferential homing to peripheral tissues like the bronchoalveolar space of CVID or the synovium of rheumatoid arthritis patients. Therefore, as a result of the close resemblance to the gene expression profile, phenotype, function and preferential tissue homing of murine B1 B cells, we suggest that CD21low B cells represent a human innate-like B cell population.

Changes in circulating B cells and immunoglobulin classes and subclasses in a healthy aged population

The study of 87 adults of different ages, including 15 centenarians, selected for their healthy status, showed that profound changes of humoral immunity occur throughout life. In particuIar, a statistically significant age‐reIated increase of the serum level of immunoglobulin cIasses (IgG and IgA but not IgM) and IgG subcIasses (IgGI, 2 and 3, but not IgG4) was detected. A parallel age‐related decrease of circuIating B cells was also observed. The hypothesis of a complex derangement of B cell function and/or compartmentalization with age is put forward, together with the proposal that healthy centenarians (as representative of successful ageing) may be helpful in identifying the physiological age‐reIated modifications of the immune system.

Soluble BAFF Levels Inversely Correlate with Peripheral B Cell Numbers and the Expression of BAFF Receptors

The TNF family member protein BAFF/BLyS is essential for B cell survival and plays an important role in regulating class switch recombination as well as in the selection of autoreactive B cells. In humans, increased concentrations of soluble BAFF are found in different pathological conditions, which may be as diverse as autoimmune diseases, B cell malignancies, and primary Ab deficiencies (PAD). Because the mechanisms that regulate BAFF levels are not well understood, we newly developed a set of mAbs against human BAFF to study the parameters that determine the concentrations of soluble BAFF in circulation. Patients with PAD, including severe functional B cell defects such as BTK, BAFF-R, or TACI deficiency, were found to have higher BAFF levels than asplenic individuals, patients after anti-CD20 B cell depletion, chronic lymphocytic leukemia patients, or healthy donors. In a comparable manner, mice constitutively expressing human BAFF were found to have higher concentrations of BAFF in the absence than in the presence of B cells. Therefore, our data strongly suggest that BAFF steady-state concentrations mainly depend on the number of B cells as well as on the expression of BAFF-binding receptors. Because most patients with PAD have high levels of circulating BAFF, the increase in BAFF concentrations cannot compensate defects in B cell development and function.

International Consensus Document (ICON): Common Variable Immunodeficiency Disorders

The International Collaboration in Asthma, Allergy and Immunology initiated an international coalition among the American Academy of Allergy, Asthma & Immunology; the European Academy of Allergy and Clinical Immunology; the World Allergy Organization; and the American College of Allergy, Asthma & Immunology on common variable immunodeficiency. An author group was formed and then divided into individual committees. Within the committee, teams of authors were subgrouped to generate content for specific sections of the document. Content was derived from literature searches, relevant published guidelines, and clinical experience. After a draft of the document was assembled, it was collectively reviewed and revised by the authors. Where evidence was lacking or conflicting, the information presented represents the consensus expert opinion of the group. The full document was then independently reviewed by 5 international experts in the field, none of whom was among the authors of the original. The comments of these reviewers were incorporated before submission for publication.

Humoral immunity in aging.

The interactions between B and T lymphocytes, leading to the development of humoral responses, are reviewed with references to the changes occurring in aged people. Aging is perceived as a process of impairment of immune functions; it is known that T cells from aged subjects have a reduced ability to produce IL- 2. However, other functions seem to be upregulated in elderly subjects; indeed, IL- 1, IL- 3, IL- 4, IL- 6 and TNFα production are increased both in aged mice and humans. These cytokines are known to control B cell differentiation, through isotype switch and Ig production. A significant increase in IgG subclasses and IgA is observed in sera of aged subjects. This contrasts with the significant decrease in circulating B lymphocytes. The impairment of primary responses to immunization, and other aspects of humoral immunity, including mucosal responses, autoantibody production and correlations with phenotypic markers of T and B cell subsets, are discussed. (Aging Clin. Exp. Res. 6: 143–150, 1994)

Hepatitis C virus transmission by intravenous immunoglobulin

The polymerase chain reaction was used to detect hepatitis C virus infection in patients who had previously been reported to have developed non-A, non-B hepatitis after intravenous immunoglobulin infusion. Of the 33 patients with intravenous immunoglobulin associated non-A, non-B hepatitis studied, HCV RNA could be detected in 15 out of 17 patients (88%) who were HCV RNA negative prior to the development of non-A, non-B hepatitis after implicated intravenous immunoglobulin batches. Similarly, eight out of nine patients (89%) in whom no sample was available for polymerase chain reaction testing prior to intravenous immunoglobulin therapy, had detectable HCV RNA after intravenous immunoglobulin therapy with intravenous immunoglobulin batches implicated in non-A, non-B hepatitis transmission. Two of the three intravenous immunoglobulin preparations implicated in non-A, non-B hepatitis transmissions that were available for polymerase chain reaction testing also had detectable HCV RNA, confirming that hepatitis C virus is the implicated virus in intravenous immunoglobulin-associated non-A, non-B hepatitis.

Lymphoma in common variable immunodeficiency : interplay between immune dysregulation, infection and genetics

Purpose of reviewCommon variable immunodeficiency represents the largest group of primary immunodeficiency patients. The variable clinical manifestations include an increased susceptibility to chronic infections, granulomatous disease and the lymphoproliferative predisposition to develop lymphoma. This review discusses the latest insights into common variable immunodeficiency and uses common variable immunodeficiency as a model to examine the links between immunodeficiency and chronic infections in causing lymphoma. Recent findingsNewly identified disease genes within the common variable immunodeficiency population, have advanced the understanding of human immunodeficiency and the molecular basis of B-cell biology. Refined laboratory techniques have better defined this heterogeneous condition by classifying the underlying B-cell and T-cell abnormalities. New sensitive methods have also identified the presence of persistent infections that may play a role in the development of lymphoma. SummaryThere are several reasons for an increased risk of lymphoma in common variable immunodeficiency patients. These include genetics, immune dysregulation, radiosensitivity and chronic infections such as Helicobacter pylori, human herpes virus type 8 and cytomegalovirus. Chronic infections may enhance the development of lymphoma in an antigen specific manner. The interaction between chronic infections and the development of lymphoma is still unclear but studies to clarify this may lead to prevention measures and lymphoma reduction strategies.