Expression of homing receptors on IgA1 and IgA2 plasmablasts in blood reflects differential distribution of IgA1 and IgA2 in various body fluids

Abstract

Although secretory IgA is the most abundantly produced Ig isotype, the mechanisms underlying the differential distribution of IgA subclasses in various body fluids remain unclear. To explore these mechanisms, we examined the distribution of IgA subclasses, the influence of the nature and sites of encounters with antigens, and the correlation between IgA subclass distribution and homing potentials of circulating IgA plasmablasts. IgA1 predominated in serum, tears, nasal wash fluid, and saliva; the levels of IgA1 and IgA2 were comparable in vaginal wash fluid; and IgA2 predominated in intestinal lavage fluids. Seventy-one percent of circulating IgA plasmablasts secreted IgA1. The intestinal homing receptor (HR), alpha4beta7, was expressed more frequently on IgA2 than on IgA1 plasmablasts, with no differences in the expression of other HRs. IgA subclass distribution among circulating antigen-specific antibody-secreting cells (ASC) was dependent on the nature of the antigen: following vaccination with Salmonella enterica serovar Typhi, unconjugated pneumococcal polysaccharide, or Haemophilus influenzae polysaccharide-diphtheria toxoid conjugate, the proportions of specific IgA1 ASC were 74%, 47%, 56%, and 80%, respectively. HR expression depended on the route of administration: expression of HRs was different after oral than after parenteral vaccination, while no difference was seen between HR expression of antigen-specific IgA1 and IgA2 ASC induced via the same route. The key factors determining IgA subclass distribution in a given secretion are the nature of the antigens encountered at a particular site and the site-specific homing instructions given to lymphocytes at that site. These two factors are reflected as differences in the homing profiles of the total populations of circulating IgA1 and IgA2 plasmablasts.

FIG. 1.

Differential distribution of IgA1 and IgA2 in serum and external secretions of 16 unimmunized healthy adult volunteers. The data are given as values (μg/ml) for individual volunteers (n = 10) in serum (A), tears (B), saliva (C), nasal wash fluid (D), vaginal wash fluid (E), and intestinal lavage fluid (F), and the geometric mean is indicated by a longer line. Note that serum, tears, and saliva were collected undiluted and these values represent actual concentrations, while nasal, vaginal and intestinal lavage fluids provide only the distribution of the two subclasses, as the collection of lavaged samples implies an undetermined dilution. Statistically significant differences between concentrations of IgA1 and IgA2 in paired Student's t test are indicated by asterisks (***, P < 0.001; **, 0.001 < P < 0.01).

FIG. 2.

Arithmetic means (± standard deviations) of the percentages of IgA1- and IgA2-secreting cells in the peripheral blood of healthy, unimmunized volunteers expressing the gut homing receptor (α4β7), peripheral lymph node HR (l-selectin), and skin HR (CLA). At least 1.2 × 10⁶ PBMC from each volunteer were assayed. The numbers of volunteers from whom the data were pooled are given under the bars. Statistically significant differences between IgA1 and IgA2 ISC in paired Student's t test are indicated by asterisks (**, 0.001 < P < 0.01).

FIG. 3.

Arithmetic means (± standard deviations) of the percentages of IgA1- and IgA2-secreting cells in peripheral blood expressing various lymphocyte markers. The numbers of unimmunized healthy volunteers from whom the data were pooled are given under the bars. Statistically significant differences between IgA1 and IgA2 ISC in paired Student's t test are indicated by an asterisk (*, 0.01 < P < 0.05).

FIG. 4.

Arithmetic means (± standard deviations) of the percentages of antigen-specific IgA1- and IgA2-secreting cells expressing the indicated HR in healthy volunteers after oral vaccination with live attenuated Salmonella serovar Typhi Ty21a (A) or parenteral vaccination with an unconjugated pneumococcal polysaccharide vaccine (B). The numbers of volunteers from whom the data were pooled are given under the bars. There were no statistically significant differences between IgA1 and IgA2 ASC in the paired Student's t test.