We successfully cloned antigen-specific T cells from murine gut-associated lymphoreticular tissue, i.e., Peyer's patches, which are dependent upon T cell growth factor and independent of antigen for continuous growth. These clones exhibit helper activity for IgA responses to sheep erythrocytes (SRBC) and have been designated T helper (Th) A. Two broad categories of Th A clones have been maintained in continuous culture. The first group supports IgM and largely IgA anti-SRBC plaque-forming cell (PFC) responses in both normal and SRBC-primed splenic B cell cultures, whereas the second group supports low IgM, IgG1, and IgG2 and high IgA PFC responses. Subclones derived from single cells maintain the parent helper properties when propagated in culture for long periods (greater than 7 mo). Cloned Th A cells are antigen specific and do not support polyclonal or immune responses to other thymus dependent antigens in normal B cell cultures. Th A cells require full histocompatibility for helper functions because addition of cloned Th A cells to B cell cultures from other H-2 types does not result in IgA responses. Cloned Th A cells are Thy-1.2+ and Lyt-1+ and Lyt-2-, Ig-, and I-A-. Th A cells bear Fc receptors for IgA and do not possess receptors for IgM or IgG isotypes. Thus, T cells that primarily promote IgA isotype responses have been isolated in high frequency from murine PP, an anatomical site of major importance for induction and regulation of the IgA response.

Two major types of T cells that deliver help (Th) 1 for B cell responses to thymic-dependent (TD) antigens have been described. The classic Th cell responds to carrier antigenic determinants and generates help for B cell responses to haptenic determinants (1-5). Induction of carrier-specific Th cells requires histocompatible accessory cells (6-8). MHC requirements for Th-B cell interactions, however, have been controversial (9-16). Studies with cloned murine Th cells (17-19) have revealed that activation of small, resting B cells requires the help of MHC-restricted, antigen-specific cells (20,21). Once activated, B cells can be induced to divide and mature under the influence of T cell-derived B cell growth and differentiation factors. It has also been shown that activation of Lyb-5-B ceils requires MHC restriction and hapten-carrier linkage for Th-B cell interaction (22, 23), while activation of Lyb-5 ÷ B cells is MHC unrestricted and can be triggered by hapten and carrier on separate molecules (22,23).Another class of Th cells appears to depend upon B cell immunoglobulin (Ig) determinants for induction. A subpopulation of helper T cells that recognizes Ig (Thlg) and normally acts in synergy with carrier-specific Th cells, cannot be detected in anti-u suppressed mice that lack Ig + B cells (24,25). Thtg cells may express two receptors, one for idiotype and the other for antigen (26). These T cells interact with B cells via idiotype recognition and require antigen stimulation before collaboration with B cells. Induction of Thxg cells in an Igh-rectricted environment leads to efficient help of B cells bearing the matching Igh-linked product (27).

In this review, we have emphasized: 1) bacterial lipopolysaccharide (LPS) involvement in IgA responses to orally administered thymic-dependent (TD) antigens; 2) characterization of Peyer's patch (PP) lymphoreticular cells; and 3) gastrointestinal immunization with gram negative pathogens and anti-LPS immunity to infection. Gut LPS, which interacts with PP lymphoreticular cells, is a major determinant for host responses to orally administered TD antigens. Bacteroides species are the principal microflora present in the gastrointestinal tract and our studies with phenol-water LPS extracts from Bacteroides fragilis indicate that both polysaccharide and lipid A activate lymphoreticular cells. The B. fragilis lipid A moiety, like that derived from E. coli and Salmonella LPS, induces B cell mitogenic responses in cultures from LPS responsive mice, but does not stimulate C3H/ H3J B cells. The inability of lipid A to stimulate gut-associated lymphoreticular tissue (GALT) cells of C3H/HeJ mice results in the induction of greater T helper cell activity in this tissue in response to orally administered TD antigens and ultimately results in an elevated IgA response pattern. Murine PP contain accessory cells (approximately 1% dendritic cells and 6-8% macrophages) and lymphocytes T (35-38%) and B (40-42%). Recent studies with antigen-specific T cell clones from C3H/ H3J PP have resulted in the isolation of IgA isotype-specific T helper cells (PP Th A cells). PP Th A cells are antigen-specific, bear Fc alpha receptors, and require H-2 histocompatibility with B cells for helper activity. PP Th A cells most effectively collaborate with surface IgA (sIgA)-bearing B cells (IgA committed B cells) for IgA isotype responses. Other studies have shown that PP dendritic cells and T cells form clusters when stimulated in vitro with sodium periodate and that these clusters promote polyclonal IgA responses in B cell cultures. Polyclonal IgA responses in cultures containing PP cell clusters from C3H/ H3J mice are considerably higher than those in identical cultures from LPS responsive mice. In other studies, the environmental influence on GALT B cells and their resultant commitment to IgA isotype is under investigation. CBA/N, X-linked immunodeficient (xid) mice possess an immature splenic B cell population which cannot respond to thymic-independent class-2 (TI-2) or certain TD antigens. However, GALT B cells of xid mice possess a mature Lyb-5+ B cell subpopulation capable of both TI-2 and TD responses.(ABSTRACT TRUNCATED AT 400 WORDS)

The induction of immune responses to orally administered trinitrophenyl (TNP)-haptenated Streptococcus mutans and its enhancement with muramyldipeptide (MDP), peptidoglycan (PG), and concanavalin A (Con A) were investigated in lipopolysaccharide (LPS)-non-responsive C3H/HeJ mice and the syngeneic, LPS-responsive C3H/HeN strain. Both mouse strains manifested similar immune responses, primarily of the IgM isotype, after a single gastric intubation (GI) with TNP-S. mutans. However, when groups of animals were first carrier-primed by GI with S. mutans for 2 consecutive days, followed by a single GI with TNP-S. mutans 1 week later, C3H/HeJ mice gave a significantly higher (P less than or equal to 0.01) splenic IgA anti-TNP plaque-forming cell (PFC) response than identically treated C3H/HeN mice. Furthermore, saliva, urine and serum from these C3H/HeJ mice possessed high levels of IgA anti-TNP antibodies as determined by the enzyme-linked immunosorbent assay, whereas C3H/HeN mice exhibited low antibody levels. Oral administration of Con A (either 250 micrograms or 500 micrograms/mouse) or purified PG (1 mg/mouse) at the time of TNP-S. mutans immunization resulted in significantly (P less than or equal to 0.01) enhanced splenic IgA anti-TNP PFC responses, especially in C3H/HeJ mice. On the other hand, MDP promoted IgA anti-TNP PFC responses in LPS-responsive C3H/HeN mice but did not augment responses in C3H/HeJ animals. A similar immune response pattern was seen when antibody levels were measured in serum, saliva, and urine of both mouse strains. These results demonstrate that haptenated S. mutans is a good antigen for the induction of high IgA responses in orally immunized C3H/HeJ mice and that this high response can be enhanced with the adjuvants Con A and PG. However, MDP is ineffective in C3H/HeJ mice but enhances IgA responses in normal LPS-responsive C3H/HeN animals.

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